I am currently using Vista Business (just the regular version) with 4gb and it goes well. Certainly faster than some other computers I've used. Can't wait to upgrade to 64bit to when all the problems are ironed out so I can take full advantage of the 4gb.
Yeah, four gigs worked great for me on Vista Ultimate and it'd do the same for Business. If you don't want to pay that much, though, grab an old flash drive and try using ready boost - it doesn't give quite as much of a boost, but it'll help.