I have the same CPU you as that system and 4gb DDR2 and upgraded to the same graphics card number, maker is Sapphire with a OCZ 550W PSU. Works pretty good for me. Starcraft II and WoW run on ultra settings with 45-60fps unless a ton of action then it goes down to 30 fps or so. Mine was a pre-built Dell Inspiron at stock.
As for heat, when running SCII for long periods of time (3-4 hours) and there's a lot of action I can reach 80c, and the system idles at 40c. That's pretty warm but won't kill your system, if you're worried about that being too hot i'd say put another fan in the side of the case (cut and make a hole for it on the panel that pops off).