For most games, there's no advantage to having 6 cores. But there's also no advantage to hyperthreading in most games.
If you have to buy right now, I would consider getting a P55 mobo and an i5-750/760 instead of the i7. If you can afford to wait a month or so, I would check the prices on the new Sandy Bridge processors before making a decision.
That being said, if your monitor is 1920x1080 (or larger), there's not going to be a ton of difference between the two processor/mobo combos. It's going to depend much more on your graphics card. At 1680x1050, you may see a little more difference, but unless the only game you're playing is Far Cry 2, you are unlikely to notice the difference, as most games should be over 60 fps, or crippled enough not to matter (Crysis, mostly).
If the above to choices are truly your only options (can't go i5, can't wait for SB), then go for the i7, but it's about $75-100 more expensive for no real boost in gaming performance.
I see you suggested the P55 FTW 200 and can't help to point-out that there is very little performance difference between a 100$ and 250$ board using the same chipset; the difference will be features. Look at what is needed and choose accordingly. For example, the ASUS Maximus III GENE could be more than enough, but save 100$ which could be put toward a higher-end GPU or even an SSD.
Zenthar, to be honest, I didn't look at the motherboard choice at all, because as you mention, motherboards have little effect on performance. I generically suggested a P55 mobo, but didn't pay any attention to the one originally suggested. The best current value would be either an i5-750/750 with non-bargain-basement P55 mobo or an AMD Phenom II X4 955/965 with an 870 or 890 GX/FX mobo (depending on how much you want to spend).
OP, you should strongly consider filling out the info requested in the How to Ask for New Build Advice thread to get a better recommendation for your budget.