I converted one of my newer of my GS3 machines today to xp64 and it was the smoothest upgrade or reinstall of windows ever. I will install GS4 tomorrow and let you guys know how it goes.

Its a
Asus P5N-E SLI mb
2 geforce 7200 cards running in SLI mode
1 RME HDSP 9652
8 gigs ram PC800 (4x2g)

I got all the 64 bit drivers and windows updates all done and now I present this lovely bucket of silicone to a cardboard box from tascam tomorrow, wish me luck

If it all goes well I will convert a second one next week