For gaming right now, there really won't be any difference between a PII 955 and an i7-920. In a few CPU-intensive games, such as WoW, you might squeeze a few extra FPS out of the i7 in a couple occasions - but the difference would still be very minimal. This is especially true if you're going with any sort of sane single card or Crossfire/SLI setup. With some higher-end cards or dual-GPU setups, like a 5970 or a GTX 285 SLI or something, then the i7 would bottleneck games less. Either way, both will give you excellent framrates in every game out there at max settings.
In the future (a few years down the line), if you choose to upgrade your video card, I would imagine that an i7 would then hold up better as the bottleneck would likely shift from the GPU to the CPU. But even by then, both processors would look ancient compared to newer technology.
I would really base your decision on your budget. Unless you really have the money to spend, a 955 or an i5-750 would be the way to go. Either one would still be better than what 99% of the PC users out there have, and better than what the vast majority of PC gamers are currently using. If you have a huge budget and won't be sacrificing the video card, then why not get an i7-920 simply for longevity?
Either way...considering it's still possible to run most games at 60+ FPS with a Core 2 Duo or Athlon x2 at decent clock speeds, you can't go wrong with an i5, PII, or i7.