I didn't find any right away that said that most software runs well on 64-bit though as usual it is more common to find articles about problems than no problems! Since this box is just about four years old now I was planning to either buy a new one sometime in the coming year or possibly rebuild this one.-J-Mac
My PC is also 4 years old and has a worse processor than yours (Athlon 64 3500+) and runs Win 7 64-bit very well, even on just 2 GB of RAM. The only software I've had any trouble with is 7-Zip, but that's actually the 64-bit edition of 7-Zip. For some reason it won't make itself the default program for any of the various archive files, so I have to open the 7-Zip GUI first, then navigate to the archive.
Anyway, I'm just chiming in to say that Windows 7 runs well even on "older" hardware. And that the 64-bit edition seems very stable to me. I can't imagine it making your system run worse, especially if you get more RAM out of it.