Like I said, you can use a GeForce2 Ultra 64MB or the ATI All-In-Wonder Radeon 32MB..and maybe the GeForce3 (you might want to ask around and see if anyone's gotten this config to work).
Personally I use the ATI card and though I don't play games often, I did play Deus Ex at 1600x1024 full screen res (the game supports it natively) all the way through a couple months back. It was smooth as silk and the graphics were beautiful. If you're a light gamer like me, the Radeon or GeForce cards will be more than enough. A lot of cards are supporting the SuperWide res of the SGI 1600SW now so finding a high quality card capable of powering resource-hungry 3D games shouldn't be that big a deal (not like a year or two ago, when the G400 Max was the only decent compatible card!). If you do a lot of professional work or even just web browsing, you'll LOVE the 1600x1024 res since it allows you to have two browser windows open side by side at the same time.
Since I currently have two SGI 1600SW units, I'm not looking to upgrade anytime soon. I considered getting two of the new Sony 18" LCDs ($1600 each) and running them in a dual setup but I decided the larger res of the SGI flatpanels was just too much of a convenience to give up. Now if only the 22" Apple Cinema display were PC compatible..