I would use 100GB or 120GB for win7 but I install a lot development software. If you have vista running on the computer, win7 will probably install easily, and the drivers might even be better. It will ask you which drive and partition to use for the install, and this is one place where a mistake can be fatal -- make sure you know the drive number and partition to use. It will detect any xp or vista preexisting and set up dual boot automatically, and the boot manager can be managed from win7 and vista. Install the 64-bit version if you can and perhaps add more memory.
For some of my installed software I have avoided reinstalls by just creating a link to the existing exe in win7, and copied the c:\user\username\appdata\appname\roaming data from vista to win7. Some of the better programs don't infest windows too deeply and are portable this way. I'm surprised by how many programs still work ok doing this. Anything that depends on the registry or installs a lot of dlls can't be copied or linked to.
I use bootitng to image partitions and it's really solid and cheap..