I looked at W8 and ran it's compability scanner. It says it is not compatable with Guitar Rig and a few other things I don't remember... now it could be that I need to update GRig... still, if you use it I would check it out.
IMO, if you go 64bit, go ALL 64bit... and by all means... do get an Octa-Capture. YMMV. You can keep X1 alongside X2 just in case. I wouldn't overwrite X1 or anything.
Good luck to you. Yeah, I like X2.
J