It depends on what you do.

If you do Websites only, then you would need just the Design package. Dreamweaver, Photoshop, and Illustrator (sometimes Flash) are all I use in design.

the Master collection has Video editing (Premiere) and motion graphics (After Effects) tools which are fun to play with, but you have to think, would you really use them? Probably not. If you think you might, then download the 30 day trials on their site and play with them before you commit the extra money towards it.

Keep in mind also that CS5 is due to be released sometime this year, I think.