The GTS 250 is basically a revamp of the 9800 GTX+ and not what you want or need in a gaming rig. Who builds a Corvette and then puts 13" temporary tires on it?
You want 64-bit Win7. Tell the idiot who supposedly knows what he id doing to shut up and build what you tell him to build.
I've seen way too many of those guys who ask what you want and then either build what they want or use whatever parts they have they want rid of.
And, yes, 6GB with the 64-bit. The 1366 i7's use tri-channel memory, so you want three sticks of 2GB in it.