1) Suppose FX is on average 10% faster than P4EE for gaming. The difference between playing a game a 60 fps and 66 fps isn't going to hurt you, and you can expect the difference to be less than 10%.
2) Now suppose that P4EE is faster by 10% on rendering. That saves you 6 minutes, if it takes an hour to render on FX. The gain here would probably be more appreciated than a gain in frame rate.
Caution: If you leave your machine on overnight, then time savings of 10% does not matter much, unless you render 24/7. So we would be more concerned about gaming, and that means FX is preferred.
Also, if you plan on rendering and gaming at the same time, ex: if you render 24/7, then P4EE will provide better gaming performance and would be the better choice.
Personally I would get A64 3400 because it is cheaper than FX, and doesn't require registered RAM. And the performance difference between it and P4EE and A64 FX is not worth the price difference. But since you are allowed to get the best processor, get the FX. Rendering at 64 bits in the future should theoretically push it faster than the P4EE. Rendering and gaming at the same time = doing half a**ed job on both.