Hi there,
From personal experience I would advise you to bite the bullet and go Windows 7 64 bit.
Of course, it is a long time since I played with (swore at really) XP 64 bit, so things may have improved, but when I tried it, I found that a lot of hardware and software just didn't work properly. I seemed to spend a lot of time trying to resolve glitches and other annoying issues and found I was producing less because of them.
Windows 7 64 bit is totally different and I had no issues with any hardware/software running in this OS.
I am now running Windows 8 Pro 64 bit and despite all the negative press I have found that the actual OS (once you get past the startup interface) is really solid. I have abused the system in many ways and have not yet been able to make the system fall over, except when I have done something really stupid. Even then, it has managed to recover itself, with very little input from me and I have nothing but praise for the way it handles everything I have thrown at it.