I can give you my build, but it was under 800 because I bought everything on sale. However, components seem to be more expensive in Canada than in USA, so you might even end up saving more. I also bought these parts over two months as they went on sale.
I'm able to play stuff like BF3 on ultra at 1920x1080, with anti-aliasing deferred turned off, with above minimum (30) fps. Playing MW3 at high shouldn't be a problem, so you can go for a cheaper gpu than the one I have so you have extra cash for the 2500k and an aftermarket cooler.
You'll have to ask someone else about an afterkmarket cooler.
Stores: That I know of in u.s.a, newegg and ncix.com. make sure you include shipping in your final budget, if you want to.
CPU: i5-2500 (you can go for the 2500k since you want to overclock)
GPU: Sapphire (or any other brand) Radeon HD 6850
RAM: 2x4GB Corsair XMS3 1600
Motherboard: Gigabyte GA-Z68A-D3H-B3, dual channel ram slot. It's ivy bridge ready and allows for pcie3, but honestly, since the ivy bridge cpus don't make an amazing difference except for integrated graphics, no point. Plus, I keep hearing that current gpus don't take full advantage of even pcie 2 x16 slots, and pcie 3 won't work unless there's an ivy bridge AND pcie 3 gpu. You can go cheaper on the mobo, I should have done that.
PSU: Corsair TX750 (you can go for lower wattage = cheaper since I bought it to allow for crossfiring when I buy a second gpu, but again, you'll be overclocking so this may be good. Google psu calculators and use several of those, make sure you indicate "overclock")
HDD: Seagate Barracuda 500GB 7200RPM (ST500DM002)
Case: Thermaltake Commander (VN400A1W2N) (plus extra cash for additional fans). It looks gorgeous and I'm glad I bought it. I just have complaints about the two vents at the top, where there's only room for one fan, and the trouble of trying to install the front fan (although, these are one-time things). You can also go for the Cooler Master HAF 912, the other case I considered buying.