The i5-750 outperforms the i7-920 for gaming, at a far lower cost. The i7-860 is even more superior. Unless you're in for extreme overclocking, the 920 offers poor value. The P55 chipset is a lot cheaper as well.
Do note that this requires you to change to 4GB of RAM, as P55 doesn't support tripple channel RAM. However, that's still more than enough for gaming.
That RAM isn't all that great. Look for 8-8-8-24 or even 7-7-7-20 timings. G.Skill has some nice sets, for example.
Can't help you with the sound card question, sorry.
It will allow upgrading to gulftown indeed, which is great. However, that will be horribly expensive (I've seen $700 as a retail price being suggested). Maybe you could consider AM3/965? The Thuban (amd 6-core) will be more affordable I guess.
To sum it up:
-i5-750 for extremely good gaming performance at a reasonable cost, but not upgradeable without changing the motherboard.
-i7-920 for the same gaming performance at a much higher cost and power consumption, but great upgradeability.
-AMD 965 for great gaming performance and great upgradeability at a moderate cost.
With upgrading in mind, I'd say:
AMD 965 for price/performance
i7-920 if price isn't an issue.