If you change the motherboard
You may not be able to reload the Vista system on it from scratch if the need arises. Also need to check if it's a BTX instead of an ATX case if you want to put a new motherboard into the same case. BTX is a reverse board from the ATX. Some OEM's like to use BTX case and motherboard since it's NOT industry standard and keeps customers coming to them for a new motherboard.
You can search for memtest or another test program for the RAM. I'd open the case up and make sure the RAM is fully seated still, it might have come loose.
If you run DEFRAG set to check for bad spots you can check the drive. Also drive manufacturers have testing programs on their sites you can download to test the drive.
How much could you upgrade that system anyway? From an x2 to an x4? Maybe add another 1GB of RAM? Cruise the computer and networking section of EBay to get ideas of what you really want to do, and if maybe you are better off sticking with what you have and solve whatever problem you currently have with it.