I have a couple of PCI geForce2 cards (for second monitors) and they seem to work just fine. I used one as my primary card for three days when my AGP card freaked, and while it didn't blow me away, the performance was acceptable. I would imagine the gf4 to be a similar story.

I'm just amazed someone's turning an Optiplex (we call them Craptiplex's here at a mostly-Dell-shop) into a gaming machine. Bravo!