as fast as technology changes you'd spend a fortune buying the new stuff all the time. pick something thats a year or two old technology wise most that stuff is fine, except for some of the popular laptop chips as mentioned earlier.
anything pentium I or pentium II is way to old to screw with if your buying something. pentium III stuff is outdated too but can work for basic stuff and run windows xp. i have an old dell pIII 550....it's not much, but still works and is my spare pc.
i'm more familiar with intel stuff than AMD, i can say that i have seen on e bay refurbed dell dimension computers with pentium 4 and 2 ghz processors for 4 or 500 depending on options. anything p-4 should work fine, and as long as it has a minimum 256 memmory, and a hard drive of 20 gb or more, that works for most basic stuff...
the secret to buying a pc or building one is building what you need. you don't need a 300 dollar video card if you don't game, etc.....
look at some places like
www.newegg.com to even get an idea for building your own tower computer, it's not that hard and when your done with it you know how to replace and upgrade things. you'll never have to buy a computer again, just keep upgrading the parts into the case....if you can build a model or rip a car apart you can build a computer...minus the floor jacks and sledge hammers
