Upgrading to Win7 or Win8
There are several factors to consider. What programs you are using? Are there new drivers or patches for migrating to a new OS? Are your motherboards 32-bit or 64-bit?
Skip Vista altogether. I ran the Business version for awhile, and I had little trouble with it, but my move to Win 7 Pro was seamless. If you decide to do dual-boot for some programs, make sure you do have separate physical hard drives for that process. They changed the boot loader sequence and file structures between XP and Vista/Win7, and trying to install on logical drives instead of actual separate physical drives will cause really serious boot headaches. (I got to the point I kept my install DVD in the drive just in case I had to rebuilt the MBR - master boot record. Even with EasyBCD installed.)
If you are doing something with video, CAD, cartography, illustration, etc, you are going to want both extra RAM and a 64-bit board so Windows makes use of anything over 3G of RAM.
For myself, there's Leading Edge and Bleeding Edge in technology. Windows 8 has just come out, and there will be bugs. Unless you are in a hurry, I'd say pick Windows 7, and get the Professional version at a minimum. Home versions come without many networking features, so stay away from that, and the price difference isn't worth the headache when trying to create a simple peer-to-peer home network.
If you are running 64-bit motherboards (and thus the 64-bit Operating System as well), scoping out the hardware drivers will be important. Most of the older drivers are integrated into the library CAB files that come with Windows, but they are not necessarily the ones that work best or even very well. A few video cards back I had a ATI card from a licensed OEM, and neither the Windows nor even the ATI website drivers worked as well as those on the original CD. And this warning includes drivers not only for things like webcams, printers, but also for internal cards, such as for video, music, etc. It's work to do this, but effort before hand will save you headache later. Put any patches or updates onto a flash drive or CD before you upgrade, so they can be installed BEFORE you install any applications or games.
** Back up your data before doing anything.** It seems so basic, but so many forget it. Turn on all hidden and system files, so that the backup grabs those files, too. Grab not only the stuff in your profile, but also the Program FIles, as there may be data files, settings, etc., that you will want to have available. Not all programs store files in the profile area, especially those running under the older XP.
When you do your OS change, don't do an upgrade. Get a full install version. That way, if your OS gets corrupted for whatever reason, you won't be left hanging. If anything happens and you need to re-install Windows, a full install will work, but an upgrade disk may not (the upgrade disk looks for files it may not find due to virus corruption, etc). Plus, you won't have to keep the old OS directory on the hard drive.
If you play with Windows 8 and decide you like it, I would seriously recommend waiting until there is at least a Service Pack upgrade available. That means others have suffered in your place, and Microsoft has fixed the most egregious errors (though some continue to elude them since XP -- such as when DirectX gets corrupted; DirectX just checks the registry to see what's installed, and instead of offering to re-install anyway, says it's already installed and quits. Really, Microsoft?) In any event, that Service Pack may or may not happen before your XP is finally end-dated. (Microsoft has been trying to end-date XP for years now, with little success.)
So, there's my two cents. Good luck.