Definitely the video card. (I know it's already been said, but I'm just agreeing)
I have a GeForce2 GTS 64 and have everything turned on and set to high and it works pretty well at 1280x1024. Although it's not top of the line, I can live with it.
If you don't want to spring out for a new vidcard you could always try setting your graphics even lower. Lowering the resolution usually helps.
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