I think, and others can correct me if I'm wrong, that the ratings of the clubs are more important than the balls. The balls are sort of % boosters (i.e. a ball rated with "3" spin will give you 3% more spin), but the clubs are the predominant factor. I think a club with 3.0 spin and a 2.0 ball will give you more spin than 2.0 clubs with 3.0 balls, if this makes sense.
My personal opinion is that wedges are invaluable since these are the clubs you can really score with, especially on par 5s and saving shots around greens with pitches, flops, etc. so that's why I would upgrade those first before buying balls.