You should see a big improvent if you upgrade to 4 GB of ram. In windows you should get a 15% speed boost on top of that. Your graphics card isn't the greatest, but it isn't terrible. It just can't handle the game properly at your native resolution, so you could try to bring that down. I have a 2.66 ghz iMac with that graphics card and 4 GB of Ram, and I get about 35-40 FPS in windows on medium, at 1600x1050 res.
I've played through most of the single player campaign by now, and the only real trouble I have is the loading screens, which take about 3-5 minutes to load (just a guess).
As long as I've got nothing else running, the missions run just fine, no lagging, no problems at all, except for a little bit right as each mission starts.
Anyway, I thought that you couldn't upgrade the RAM on the aluminum iMacs w/o taking it apart. Am I mistaken?
EDIT: Just found the guide on how to do it on the Apple website. Much easier than I thought it would be.