I don't know how much you're intending to hang onto from your current system, but it sounds like not much.

Given that, I'd buy a complete system from someone. Probably Dell. The me from five to ten years ago would think I was an idiot, but, honestly, these days the prices for pre-built systems are usually very competitive, the components are quality, the system has been verified to be internally compatible, the noise levels are usually very good, using custom-made components that you can't really duplicate on a homebrew, and the support tends to be pretty solid. These things definitely did not use to be true.

There's not really a 4GB limit on 32-bit operating systems, but a 4GB limit on 32-bit non-server-class Windows operating systems. That may be the same thing to you, of course. There will be an x86-64 version of Windows 7, but there will also be an x86 32-bit version. And, yes, the i7 is 64-bit.

I have no information on video cards or power supplies; sorry.