The only benefits you'd see from going to Vista 64 on the current hardware would be support for up to 8 Gig of RAM on your specific motherboard, and DX10 functionality with some of the newer game titles. I doubt very much if you'd see any improvement at all where OFF is concerned.
I certainly wouldn't go exclusivley to a Vista based system, be it 32 or 64bit.....period. The only thing I might recommend would be to grab yourself another hard drive and go to a dual boot arrangement. It's not all that complex a project as long as you're reasonably saavy with PC's in general, and it would give you the best of both worlds. In fact, I've been contemplating doing that myself. The only thing holding me back is the cost of Vista 64....it's not exactly cheap, but hard drives are nowadays. At that point I have to ask myself the question "Is the addition of more system memory and the eye-candy of a couple of DX10 games worth that kind of investment???" In my particular case the answer is "probably not". Only you can decide, but in your specific instance I'd probably stick with XP for now.
One thing puzzles me quite a bit. What is it that makes you feel your system isn't performing up to it's full potential? That's a fairly slick rig you're running there. It should run just about anything I can think of quite nicely, bearing in mind that OFF is running on only one of your 4 processor cores and is probably utilizing far less than even a single Gig of RAM. A 64 bit OS and more RAM aren't going to help in that department....not in any real significant way. Bear in mind also, this is only one man's opinion. Some may beg to differ, but that is sincerely the best advice I can offer with any real degree of comfort.
I think there are a number of us eagerly awaiting Billy's next operating system rather than a dressed up version of XP, which IMHO is exactly what Vista is. One thing I do know. The taylor made a very expensive suit there......lol.
Cheers,
Parky