Well i can speak from experience that Photoshop might give you some trouble installing on 64 Bit as we have had that question brought up a couple of times here already.
But my biggest suggestion is to partition and run the OS's on 1 drive and use the 2nd drive as backup/storage. I currently have 3 drives and my setup is similar to that. I have all my real data on the drives that are not connected with the OS's i run. This way if i have to format, like i do this week, then i do no lose anything. But i always have the information.
I also use a external which is my main backup source.
Veedub...he has Vista 64 already. got it from school for $10.