For those on a restrictive budget who just need a good computer and dont want to hand build their own, there is no beating the Emachines $399 el-cheapo (I prefer the T2341). You get a damned fast computer that only needs a $100 graphics card to scream (which you may already have in your current puter). It also helps to upgrade the ram a tad as the 128 it comes with is a bit anemic...but that can be done for under $50.
So worst case scenario, you spend $550 and get a machine that while isnt the fastest on the planet...will run circles around an old ~1 gHz machine. Now I know that $550 is a whole lot of money to some folks, but for most it is a doable thing, especially if you are motivated.
You can build a cheaper puter, but for those less adept, you cant beat the emachine with a stick......just remember, its got a pretty small power supply...so dont try and stick 3 hard drives on it....you PS wont last long if you do:D