the main differences between the expensive (professional), and cheaper versions (movie studio HD, movie studio HD platinum, movie studio HD platinum production suite) of sony vegas are the amount of layers you can use. All versions are limited to 10, apart from professional which is unlimited. For a standard movie you might only need to use a video layer, and a text layer.. so you'll be absolutely fine with the cheap version
you can see the differences between all version @
http://www.sonycreativesoftware.com/...udiohd/compare
Good luck! :-)