Funny enough I just upgraded today, I had bought an extra 4GB of memory last week ($25AR from newegg, just couldn't resist!) and thought I'd give it a try.

I installed Vista64 from scratch, I've never been a fan of OS upgrades since it always seems to have small problems. It took about 30 minutes to install, pretty much on its own until the end.

Running Octopus now on it, no hassles and Octopus is connected to my 2nd XP machine without a problem (I did have to run octopus as admin). I haven't noticed any kind of performance boost, but thats to be expected since I run dual core which is at 100% cpu pegged -- I really need to upgrade to a quad core I get 30-45 FPS outdoors, and 6-10 fps in dalaran (sigh), which is exactly the same in XP.

FWIW If you own any copy of Vista you can call Microsoft and they will give you the serial number for the 64 bit version and de-activate the 32 bit one (or its the same number, dunno exactly). You still need the install media of course, they charge you if you want that. I had a free Dell vista32 serial that I never used, I called them up and they gave me a 64 bit version -- I already had Vista64 DVD from work though so I didn't have to get that.