Somebody correct me if I'm wrong, but I think 2560x1600 is the highest current graphics cards can support.
That's only the resolution for the highest standard for monitors that use consumer graphics cards (I think, I really don't know a lot about monitors). Hook up three 2560x1600 montors using eyefinity and that's 7680x4800. However, even my 5970 would have a bit of trouble running anything really demanding at that resolution.
Oh, and about the price:
My comp has basically the same specs, except I didn't buy a monitor, keyboard, or any other peripheral, just what's inside the case. Also, my graphics card is the dual-gpu version of the 5870, the 5970, and it was $300 more. All of it totaled together was $2000. His, with the peripherals and the lesser (although it still rocks) graphics card comes out to $2598 if this currency calculator is correct.
While I can see why you would go with the convenience of prebuilt, that $598 is why you would want to build it yourself.