Ubuntu – What version of Ubuntu to use for Desktop with 8Gb RAM

Yes, you should be able to without any problem. The underlying kernel is the same for both Windows 7 and Server 2008 R2. I, personally, have not had any issues when using a "Windows Vista" driver on Windows Server 2008.

That is a terrible misunderstanding. Yes, your applications might take up a little more RAM, but overall, it will hardly be noticeable; they certainly won't take up twice as much memory. No one would switch to 64-bit if that was the case.

You'll be completely fine with 2GB of RAM on a 64-bit Windows 7 machine.

Per your followup: I don't know where you heard this rumor about Windows 7 taking up twice as much memory, but it's completely false. There is absolutely no way for a 64-bit OS to take up twice as much memory as a 32-bit OS. It doesn't matter what hardware you have on your computer.

The main advantage to having a 64-bit OS is the ability to use more than 4GB RAM. On top of that, any applications written specifically to take advantage of 64-bit processors are going to be slightly faster than their 32-bit counterparts. Other than that, there is really no major difference between the two. So I would say you probably don't need a 64-bit OS, although I would still recommend installing it because there's no reason not to.