Yeah, I had this problem when playing Red Alert. It's definintly the video card. I'd get myself a better one, but I don't want to waste my money when I'm going to eventually get another computer. (I use the word "eventually" lightly)
eMac w/ Mac OS X 10.4.5, 1.25GHz, 768MB DDR Ram, 40GB harddrive, and an ATI Radeon 9200.
Dell desktop w/ 3.2GHz, possible 2GB DDR Ram, 120GB harddrive, and Intel Extreme.