If I should install Windows Vista (Ultimate) on a 500GB harddisk, can it handle the whole space as one disk or do I need to partition it in 2 or 3 parts before installing?

On a single drive you can go as large as two terabytes, so you're fine with a 500GB partition. The theoretical limit is 16EB, but I doubt that will be an issue for some time.

I have three of these 500GB buggers and one 750GB (which a friend brought along, still to expensive here ) in my Vista box.

No problems at all!

Thanks guys