Depends on what resolution you play - Your cpu will be slowing you down if you play at low resolutions (lower than 1024x768), and the video card will be slowing you down at 1280x1024 and higher. If you plan on playing high resolutions with aa/af and all possible quality settings on high, then a 6800 gt/ultra will be much faster than a 6800 or 6600gt on a 2.4ghz p4 system.
An extra 512mb ram will also help a lot with game loading times and disk-caching.