I'm here...so 300 is your budget? Are you a gammer and WANT a good video card to play BF2, QUake 4, etc?
If you want a good system, 300 is kinda low..
You need new mobo, CPU, RAM, Video card, and a good PSU to handle that rig.
On average:
Mobo + CPU combo = ~200
RAM = 80 for a 1GB in Dual channel
PSU = 60-100 for a GOOD one without a hole in your wallet
Video card = 200 for a good one
so that's about 550 right there for a good gamming system assuming you get the bestest best price. Both of my systems are pretty costly, 1600 for one and 2500 for another