Whilst it is sometimes possible to get unsupported laser printers working under XP64, with a multifunction machine you're basically screwed as a: it doesn't support postscript, so the generic PS driver won't work and b: the scanner driver would also be broken.
Frankly, I suspect the only reason why you're finding XP 64 to be faster and more reliable is that it's a fresh install. There are far too many gotchas with it in terms of hardware/software compatibility to make it terribly viable for corporate use. I evaluated it (and Vista x64) for our own purposes some time ago (I'm a senior manager/co-administrator of a medium sized charity and run our I.T. dept), and, after some considerable effort spent trying to get certain things working, concluded that it was completely unsatisfactory. A server OS is another matter, but not enough desktop applications/drivers in common corporate use are 64-bit aware.
Sorry, I know you don't want to hear this, but I suspect that you're only going to solve this by going back to XP 32-bit