I'd put more money in that CPU and go with 8 gigs of RAM. Right now, 16 is a little overkill and the great thing about RAM is that you can always buy more and slap it in there down the road when it's cheaper... A faster CPU, not as easy to do.

And the price difference between 4 and 8 gigs of RAM doesn't make it worth the effort.

I'd go with an Intel i5 (for lower cost), 8GB ram, and a Nvidia card... maybe a 500 series to cut costs or something in the 600 series if it fits in your budget.

I'd invest in a better GPU, games like BF3 are very demanding and Arma 2 is poorly optimized. How good do you want the quality to be?

These are my off the shelf Dell Inspiron 620 specs that I got on special off the Dell NZ website for $890 NZD:

Intel Core i3-2120 @ 3.3GHz
4GB RAM (can't remember DDR or GDDR)
AMD Radeon HD 6450 (The weakest link in the system, and no doubt will need upgrading if I'm gonna do any playing of current games)

Dunno about the rest of what's on your list as comparison to my PC.

So if you get a significantly better machine for

That CPU is fine, but wait for Piledriver if they aren't out yet. Also I'd say go for the 7850 for the GPU.