please tell me how this would all be, and if it would be good enough to run bf3 (and also how good could it run it)
and are all these parts compatible, i noticed my RAM says its for i5 and my processor is i3, tell me what you think, compatible? good? bf3 good? and remember im open to suggestions, but please keep them in my abt $600 budget...
It looks good overall, but you might encounter some bottlenecking with your CPU here. You probably won't get max settings, but it should definitely be playable. You meet almost every recommended requirement except for CPU and GPU (which make the biggest difference in games :-P). BF3 recommends a quad core CPU, and as for GPU, it wants a Radeon 6950 or better, but the 6870 is really close. You should be fine with what you have, you can probably get medium-medium high settings.
I think it'd really be worth the extra $75 to get a quad-core processor, particularly the i5-2500k that everyone loves so much. You'll be glad later if you spend the money now. (Plus they're on a $15 off special at Newegg for the next few days).
Personally, I don't think 2000 MHz DDR3 is worth the trouble. It's always got a latency of 9 or 10 ... which in terms of true speed is the same as (actually slightly slower than) 1600 MHz with a 7 latency. Considering that 1600MHz is typically way easier to set up and get stable, with less messing around in the BIOS, I'd always go that route. If you're really into overclocking RAM, then you can get more out of the DDR3-2000, but for most that's not an issue.