CPU - AMD 64 3200+
$160 Don't be fooled by the GHz, this will kill most P4s in gaming.
Motherboard -MSI K8N NEO-F
$80 Very solid motherboard
Video Card - eVGA 6800GS
$169 -$15 mail in rebate good till the end of the month. Yours will not cut it in most modern games. The video card is arguably the most important part in gaming systems, and this fits in your budget. This card has the one of the best price to performance ratios of any card out there.
RAM - 1GB Corsair ValueRAM
$70. If you could post what RAM you currently have, I can determine if this is necessary.
Total = about $480 -shipping
This would be a pretty solid build, able to last for a while.
Oh, and what kind of PSU do you have?