1) Generally speaking, if the low-level devices (chipsets, bus driving logic, etc.) are fairly different, Windows will halt on boot if you just transplant the hard disk. This has been my own experience at least.
2) I agree with KS on this one, if you have no good reason to use 64 (e.g. you do rendering/arithmatic simulations/video editing/some other application where a single process can and will use >2 gigs of memory) don't. I do simulations and as such have to suffer from some missing drivers (VIVO for 7800GT) and missing low-level apps (64-bit native Flash, 64-bit native Java plugin, 64-bit native video codecs, etc.) Yes, you can use 32 bit apps. Why do this if you will see no benefit?