Ick...a P4...
Going by the high end graphics of that era (GeForce 7800GTX/7900GTX), I'd say the best you can do with that bottleneck is a GeForce 9600GT or a GeForce 9800GT (which are similar in price and performance, though the latter is more powerful). After that, you are really maxing out on the CPU. I wouldn't recommend Crysis with it, as I have the AMD equivalent to that CPU, and it's on the bare minimum requirements for the game.
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