The thing about video is that even if you are a casual gamer, you will find that games really suck when you have low framerates.
I've been playing up till the beginning of the year with a GF3 Ti200, and it worked fine until I got Call of Duty. I played through the whole game and loved it, but at many point I would get some pretty big slow downs (played @ 1024 x 768). I also tried playing Halo, but that game was pretty much unplayable.
I went out and got a 9700pro (was also looking @ the 5900XT, but got the ATI just for a change, but kinda wish I got the NVIDIA cuz I like their driver interface better and have had better luck with stability). Anyhow, I played through CoD again and the whole game was much, much more immersive and fun. It was much easier to play, too, because I could make quick turns very accurately and aim much better and everything. I also went through and played C&C generals @ 1600 x 1200 resolution, which was cool.
Halo still sucks, but that's cuz my processor (XP1800+) can't really handle it and because I hate the stupid save points (I'm a sucky FPS player, so I need to save after all the hard parts, and I hate how Halo will sometimes save right before you are about to die simply because you cross some stupid line).
Anyhow, I'm not sure what you are using the computer for, but you have an awfully powerful system for most tasks (office work, internet, etc.) and it would be a shame (IMO) to have all that power throttled by a low end vid card.
I'd say that you should spring for a 5700Ultra/9600XT at least, but if you can swing it, then step up to a 9700pro/9800pro or a 5900XT. Those cards should last you quite some time.
-D'oh!