Lower your settings until you sit on about 20-30 fps, that's the best you can hope for with your machine without suffering to much in the visuals department. Im running a PIII800, 512Mb SDRAM and a Geforce 3 and this is as good as I or anybody else (with similar specs) can get it.
If you can't live with 20-30fps, grab your reciept and disk and return it to the store.
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Lady Astor: "Winston, if I were your wife I'd put poison in your coffee."
Winston Churchill: "Nancy, if I were your husband I'd drink it."