Upgrading your PC is something you've likely considered, and its likely a major reason for coming to these forums, so I'd like to clarify a few common upgrade questions:
1) Can I upgrade the graphics card in my laptop? Most likely, no, currently nVidia and AMD are pushing for more upgradable graphics in laptops such as nVidia's MXM, however it isn't widely adopted, and therefore laptop video upgrades are rare, and usually not worth the price.
2) I have integrated graphics on a desktop, should I upgrade my graphics? Maybe, if you don't game, there is no purpose to upgrade the integrated graphics, they are entirely fine for video playback, TV watching, photo editing, audio editing, web browsing, online games, and will actually handle a number of 3D titles. The only thing the graphics card adds is extra 3D processing muscle, which is only seen in entirely 3D applications (DirectX) such as video games.
3) Sound cards? Sound cards are more of a personal preference ugprade, there isn't any need to have a soundcard in a modern computer as the integrated is easily high enough quality (even for a home theatre system). Some users prefer a soundcard due to extra interface ports it can add (for example, S/PDIF, MIDI I/O, optical I/O, although all of these features are becoming more and more common on integrated solutions)
4) I want to make my computer faster, what should I upgrade? Well, first I'd suggest looking at what you have, the slowest component should be upgraded first, even if it may seem to be the "least important", such as a hard-drive. Also consider your operating system choice, two computers with identical hardware, one running Windows XP, one running Windows Vista, the XP machine is going to be faster for most applications as Windows XP is less demanding than Vista which leads me to the next point:
5) Vista? Windows Vista is the new Microsoft operating system, a lot of users are considering the upgrade, there is also a lot of bad press on Vista, so I'll try to clarify what the two sides are:
The side in favor of Vista is saying:
-Adds a lot of visual effects and looks nicer
-Adds DirectX 10
-New features to Windows, like Windows Defender
-Supports some games like Halo 2
The side against Vista is saying:
-Visual effects are overdone, and waste system resources
-Resource hungry operating system, uses too much HD space and RAM
-Most of the "Vista" features are available for XP
-Doesn't offer any killer apps over XP
-Lots of driver incompatability and software incompatability problems
Generally, you should decide for yourself if you want to upgrade to a new operating system, the "newer OS" point is to mark that Vista is new, meaning it has new features, but new problems, along with the lack of maturity found in XP (maturity meaning less applications are targetted for Vista, while XP has been the de facto standard of PCs for about 5 years).
I hope this clears up some questions on upgrading your system, generally I won't advise any form of upgrade unless its really needed, however this need is relative to application load, if you just bought a new video game and your current graphics card doesn't support it, I would say that you need to upgrade, however if you want to upgrade the graphics card just for the sake of upgrading it, there is no point in spending the money (at least in my opinion).
The best advice I can give on assesing if you need to upgrade, try your target application first, most programs have a demo, which exists for two reasons, firstly to let you try the software out, secondly to let you try the software out on your system and see if it will run correctly. If the performance is sub-standard, I would consider upgrading to run the application.
Below is a list of components and what performances they relate to:
Processor power (speed in MHZ/GHZ, # of cores, etc) - General system performance, number of applications you have can open at once, load times on programs (minor), video encoding (heavily), audio encoding (heavily), software compilation (heavily), video playback
RAM bandwidth (speed in MHZ) - System boot time, application loading time, processor power (faster RAM usually improves processor power), multitasking
RAM capacity (size in MB or GB) - Number of applications open at once, application load time, application performance, general system performance, video playback
Hard Drive bandwidth (rotational speed (in RPM), interface bandwidth (in Mb/s, MB/s, or Gb/s)) - Application load time, system boot time, general loading times, video playback, general system performance
Hard Drive capacity (size in GB) - Amount of stuff that you can store
Video RAM (video card's memory, in MB) - Very little (it does actually play into maximum resolution, however for the current highest reoslution commonly available on desktop PCs (2560x1600), 64MB of video RAM is sufficient to display a 2D image (the desktop))
Video Processor (the GPU itself, reprsented by a manufacturer name (such as nVidia) and model name (such as GeForce) and PR number (such as 8800 Ultra)) - 3D applications such as video games, CAD, OpenGL rendering, Aliasoft Maya, nVidia Gelato, etc
Internal busses (PCI Express, PCI, AGP, HyperTransport, the front side bus) - General system performance, you want higher internal bandwidth to remove any potential bottlenecks due to bus bandwidth limitations, additionally they influence what parts you can put into the system (for example, if you don't have AGP, you can't have an AGP card)
External busses (FireWire/Lynx, USB, PS/2, RS-232, LPT, MIDI) - What devices you can connect, some provide more bandwidth than other (for example FireWire generally is faster than USB, which is faster than LPT)
Wired networking (Ethernet) - Speed at which you connect to the network, has very little influence on broadband internet (most broadband internet is 3 to 9 Mbps, Ethernet is usually 100 Mbps, and 1000 Mbps is becoming more common), faster networking allows faster file transfers on your local network, Wired networking also provides much lower latency than Wireless networking, which results in higher performance (this part acutally DOES effect broadband, as the lower latency still helps)
Wireless networking (802.11b, 802.11g, 802.11 Pre-N Draft) - Speed at which you connect to the network, higher latency than wired, however can provide some benefits (if you're in a situation where wires are impractical) Also can provide internet services at remote locations for PDAs, laptops, etc, IF there is a hotspot (some airports, coffee shops, book stores, schools, etc)
Other interfaces (Bluetooth, RF, IR) - Provide wireless interface to some devices, usually the device is designed for just that interface, so bandwidth isn't a problem (its usually an interface device, so you're not really worrying about bandwidth). One thing to note is that all 3 of these interfaces are easily interferred with (Bluetooth to the least extent) by other thigns such as other IR remotes, microwaves, cordless phones, etc
Internet connection (Broadband (Cable, DSL, WiMAX, FiOS), Dial-up (28.8k, 33.3k, 56k), T-carrier (T1), ISDN) - Speed at which you connect to the internet, you could have the fastest computer in the world, if you have 28.8k it will be a slug on the internet, influences anything related to the internet (instant messaging, e-mail, any streamed online content (movies, music, games), downloads, web browsing, etc) the faster the connection the better.
I hope this helps to clarify which parts or interfaces effect what aspect of system performance, and can help target what needs to be upgraded to attain the performance levels you'd like.