However, if you're just gaming, the i7 is almost certainly not worth it, especially if your budget is about $1000.
Places to improve the build: CAS 7 RAM, like this G.Skill Ripjaws 4 GB kit would be better than the CAS 9 RAM you have selected, for about $10 more.
Depending on the resolution of your monitor, buying a better graphics card would improve your gaming experience more than anything else. If you're willing to spend up to $200 more, instead of buying the i7, you would get a lot more gaming performance out of buying a better graphics card, like this ATI 5850.
Without knowing more about your budget, resolution of your monitor and intended purpose for the machine, that's about all I can tell you right now.
Recommendations stand, then. Assuming your budget is about $1000 and with the Intel requirement, I'd go i5-750 and get a 5850.
It's probably cheaper to buy a somewhat future-proof motherboard, rather than buying a new one along with a new CPU. With that said, you might consider an i7 a slightly more future-proof build, but I don't think it's worth the extra $200 just for that reason alone.
You could get even more bang for your buck by going with an AMD Phenom II processor and if you can find about $100 in savings between processor/mobo, you could get a ATI 5870, which will pretty much max your gaming performance. Furthermore, the AM3 motherboard is the most future-proof option out there right now.
Do you need a case or an OS, or is that already covered? Do you need a hard drive?
The power supply appears to be $130 on newegg right now, this Antec Earthwatts 650 is $90, or $80 with a promo code that expires on 3/16 (Tuesday).
The most recommended hard drive (at the 500 GB size) Seagate 7200.12 for $60. Alternatively, the Samsung Spinpoint F3 and the Western Digital Caviar Black WD1002FAEX (technically a 1 TB drive), but the Samsung doesn't appear to be available on newegg.ca right now.
Even when using a 5850 you won't have to worry about bandwidth issues when going XFire on a P55 platform (x8/x8 constraints). The difference is only noticeable in benchmarks, so you won't see any degradation in real world performance (like gaming) unless using higher end cards. Also, you might want to look into a motherboard with SATA III and USB 3.0 capabilities. The ASUS P7P55D-E PRO is a great board for $200, or the GIGABYTE GA-P55A-UD4P at $200 as well. Also, if going with the i5 system, you definitely want to get an aftermarket HSF since the stock cooler for the i5-750 is garbage. The Cooler Master Hyper 212+ is a great cooler for around $30.
I have battled over the intel and amd thing for a while. I've owned both, and althought for gaming alone amd seems to be better, i do a lot alt tabbing in games and my last intel seemed to handle that better for running a game with a few browsers open and music playing. Though i dont know, is AMD rocking hard enough to not bother spening the cash on intel?