For about $300 you can pick up a Geforce2 GTS, or an MX. Of course, the prices will be less than tat! However, the GTS will pack a bit more punch, although you can save some cash and go for an MX. With the card manufacturers though, I'd go with one that updates their drivers often, and comes with decent software. For instance, creative's Geforce2 cards are great (although a tad more expensive) and offer good software and support.
The bigger your hard drive, the sooner you'll need to add more space!
WBK38 w/Sanyo Denko 92MM fan
Asus A7V133 rev7
Quantum 7200 ATA-100 20GB
Fujitsu 5400 ATA-66 10GB
SB Live MP3+ 5.1
Creative Annihilator 2 GeForce2 GTS 32MB