Zolgar: Another thing to watch for on the 'buy and upgrade', it's not uncommon for inexpensive computers to have proprietary motherboards, that are only built to house what the computer needs. I once had a Dell, opened it up to put in a new hard drive, and I had to do a PCI card, because while there was a -spot- for an AGP slot, there was no AGP slot (this was in the days before PCI-E)
Blue, what happened leading to computer building failure. if you ping me, I can help you wit the basics of building your own rig.
cjrgreen: That still happens, sometimes, on low-end computers. I've encountered current mid-tower units with no PCI-Express slot (or just the solder pads where one should be). If you're considering a buy-and-upgrade, you need to do your homework.
Also, with a buy-and-upgrade, you probably will not be able to overclock, and you often will not be able to upgrade the CPU.
Not overclocking is fine though. I never overclock my rigs, don't feel the need to.
And in general, when building a PC, or buying with intent to upgrade, I find it best to not even really plan on upgrading the CPU. CPU/Mobo is the most expensive single (yes, I know it's 2 parts technically, but they're two you have to be specific about and get ones that work together ;p) part of a computer, and so I personally prefer to spend more there on something that will last me a while.
Upgrading RAM, will cost you under $100 usually. Upgrading video will usually be $100-200 depending on how good you want to go. A new hard drive? Those get cheap and cheaper and cheaper. But upgrading your processor will cost you at least $300 usually.