heya martin.
CPU: The i7 is a fantastic cpu, but to be frank, it's way too damn powerful for just gaming. If you can get an i5 instead, it would save you a decent amount of cash without cutting out much performance.
PSU: If you're just running one gpu card, you don't need 650w of power. The extra headroom is nice, especially if you plan to SLI stuff later, but for a one gpu build, it's overkill. could probably go 500w, maybe even as low as 450w, but that's a little risky, and having more is always better then having less when it comes to psu's.
GPU: I'd say a gtx 260 or 275 for nvidia, or a 5770/5850/5870 ati card would do you well. Both brands make excellent cards, but it's your call (5770's are dirt cheap and comparable to gtx 260's).
Here's a nice complete build I threw together really fast. Edit as you see fit:
Case:
Antec 300 ATX Mid-TowerMobo:
MSI P55 LGA 1156 ATX MoboCPU:
Intel Core i5 750 2.66GHzPSU:
Corsair 650w PSURAM:
G.Skill 2x2GB DDR3 1600GPU:
2 x HD Radeon 5770 1gbHDD:
WD 750GB Caviar BlackCD/DVD:
Lite-On CD/DVD BurnerCase Fans:
3 x Apevia UV 120mm FansTotal price comes out to: $1,075.64 without shipping, keep that in mind and adjust accordingly
This should be able to handle things quite a bit better then your current build. Crossfire 5770's are comparable to a 5850 or 5870 in terms of performance (or a GTX 275 or 285), 650w PSU is more then enough for this build. Case is nice, extra fans'll keep everything cool. CPU is more then enough for gaming, you don't need more then 4GB RAM for gaming. CD/DVD burner you can get rid of if you have one, 750GB HDD is a bit more then you had, and WD Caviar Black's are pretty reliable. Also, this build is extremely upgradable if you do plan to upgrade in the future, but it should tide you over for a while. Keep in mind that mediocre performance in MWLL (30fps, High, no AA, 1680x1050) = can destroy pretty much every other game out there.
If you want a CPU Heatsink, here's a link for one at $34.99 U.S.:
CPU HS:
Cooler Master Hyper 212+Also, keep in mind you can look for combo's to save even more by swapping out similar parts. I'd like to do that for you, but combo's change alot, and I'm tired right now (2A.M. in the morning). Hope this helped!