I wonder how long your old computer will last. My AGP/478 machine failed a couple of months ago, one component after another. It just wasn't worth the money or bother to buy new components so I got a new PC.
First the PSU died, then the graphics card, and then now it's either the mobo or CPU (can't determine without another 478 mobo and CPU). I have it permanently and massively underclocked so my sister can use it. I had it since June 2004. It's still a pretty formidable machine. P4 prescott 3.0GHz underclocked to 2.0, X800pro underclocked by 40%, 2GB ram underclocked from 400MHz to 266. Can still run oblivion on 1024x768 on medium settings. Will run RTW with 3000 men no problems
I wouldn't bother buying anything to upgrade such an old system since if it craps out on you then all that cash is wasted on useless components. I spent 60 bucks on a new PSU for it and I pretty much regret it, although I can still use it to build a new PC when it dies.