Can't say anything about Vista (I had this PC built with XP pro last year), but I run a Gforce 8500 card with it's own 512 MB graphic processor, Intel quad-core processor/4 GB RAM at 2.8 Ghz and Orbiter runs as smooth as a baby's behind.
From what I gather, you need roughly 2 GB RAM to run Vista smoothly. If you've got more than that, that's gravy. If you've got 4 GB or better, you should be flying (pardon the pun) as I hear the dual-core processors are half decent in and of themselves.