How much do you have to spend?
If you have about $100 I'd go for a radeon 2600 XT. If you have a few bucks more, an X1950 PRO can be had in the $130 range, and it's alot better.
I wouldn't spend more than that on your platform, your CPU will limit you a bit. Indeed, if you want to play an RTS like Supreme Commander, you're really going to want a dual-core processor...