I started out with the Movie Studio version of Vegas (called Screenblast Movie Studio then) and quickly moved up to the full featured version.
The two biggest things you'll run into is the limit of four video and four audio tracks for Movie Studio (I have several project with literally hundreds of video tracks - needed when you use a lot a track motion adjustments and don't have the time to keep on resetting keyframes) and the lack of scripting support (I have a handful of scripts I use for almost every project).
Aside from this, you'll probably be fine with the MS version - Also, Sony occasionally has great upgrade deals (usually a couple of months after they bring out a new version) so you might be able to upgrade to Pro 9 (after it is out) for only a couple of hundred at some point - no guarantees but it has happened (how I moved up).