VirtualBox is in beta for Mac OS X as host.
I run Parallels with a cheap OEM version of XP Home and it works quite well in full screen and coherence modes (sharing content between Mac and XP including shared folders and literally copy/paste or drag/drop between OS's, super cool). If you're into more advanced software and need Direct-X support go with VM Ware Fusion as it's slightly better in that regard, otherwise the features between the two are similar. If you opt to run Vista, I suggest 4GB RAM on your Mac and set aside some addition partition space if you use bootcamp.
There is a whole forum for this kind of thing on MacRumors, check it out.
-jim