I used an 18 gig BootCamp partition to store Vista Ultimate.
I used to have a 10 gig, but I filled it up pretty quickly. I don't plan on using the partition for much other than testing now and then and keeping an eye on the competition. ;-)
(I am going to have Parallels use the BC partitition.)
BTW, Vista sucks something fierce. Not even saying that as an Apple guy..I am just shocked at how bad and buggy it is. I was unimpressed with Apple's "Top Secret" features, but at least Apple had the balls to delay Leopard for quality reasons. Vista needed another 8 months of polish, consistency, and bug fixing...