Now, for the memory, I'm in Canada and I don't think you'd be able to order the same memory that I got, but there's some RAM on sale on newegg for the shell shocker, it's probably going to be around 30-40 bucks, as it's $40 in Canada and the prices are usually lower in the US for the shell shocker
Anyway, your OS is going to be around $100, depending on whichever one you want, it isn't a big deal.
Now, just a little friendly advice, tweaking some of your hardware slightly might give you better actual performance, or perhaps better benchmarks, but the truth is, unless you plan on investing a decent amount more of money, or you plan on buying completely different products, I highly doubt you'll notice much of a difference between set ups.
PS after looking at your GFX card, that's a pretty good deal on a 6850. And, even though it isn't quite as nice as a GTX 560, it's a great card for that price.
EDIT: Sorry about that, I forgot to mention that all of those above products are from the Canadian site, which might mean that the prices listed are higher than what you can get them for. Anyway, good luck!
GTX560 is my personal opinion. I can play everything on max settings (even AA) that i own: Witcher 2, Starcraft 2, and i will be letting you know on Skyrim. ASUS makes a great 560, and they have amazing OC capability as mine is at a 950MHz from stock 810MHz. I paid $165 after MIR, and was the best thing i put in my build outside of my corsair HX750. The 960T is a great choice, but remember not all chips are guaranteed to unlock cores.
Only thing i would look at as far as a MoBo is something that can run SLI or CFX, depending on what gpu you look at. This will broaden your upgrade ability later on if you need a little bump in gaming performance. Ram is cheap, usually $50 for 8Gb DDR3. If you aren't storing a ton of information, i would suggest even going with a $40 HDD. A couple options at 7200RPM, one even with 16MB cache, not terrible. could give you a bump in the GPU you really nead. Case can be a little cheaper, if you aren't picky with looks, a HAF 912 is probably the best case for its price.
Richter makes the best point though, this budget wont give you a major difference in performance between recommended hardware, but the more you can shave off of things that dont matter, the better you can get with your gpu.
This being said, games as of now barely support quad cores, and even benches test the 955BE as the best bang for your buck with AMD's cpu's for gaming. That being said, with you on such a tight budget, i even recommend the Athlon IIx3 Rana for $75. It has the same chance to unlock the 4th core, and its rated at 3.1 GHz. The chip will run 8GB DDR3, up to 1333 i believe, and you will not see a performance drop like you would spending another $50 for the 960T instead of putting it towards your gpu. Just my thought/2 cents.
I know the processor seems to be lacking, but expecially if you unlock a 4th core, and run it to around a 3.6GHz OC, you should have plenty of juice to make use of 95% of your gpu. CPU's are cheap, and you can always upgrade, so if you really aren't happy, maybe even save up another $50 before you buy, and get a 960T or 955BE. Total cost though after MIR is:
The GTX570 will really shine, and is as good of a card as you need unless you plan on running a triple monitor, which i don't see in your budget for this build. good luck.