Until the 64bit apps arrive I doubt there is much to gain from running xp64, I've just built an XP 64 3000+ PC for a chap but he's having the 32bit XP installed.
.... XP64 Not worth the bother yet

maybe 6 months down the line.
I'm gonna use it with WinUAE ... whats AF2005?
I'm gonna use it to write ADF's to disks mainly as I prefer to use my Amiga's too.