Just watch the movie if you want. It has no bearing on the continuation of Series 3 or 4 as far as I know. Still haven't caught any of Family guy series 4 yet, but the film is just like a feature lenth episode in itself. I'd seen every Family Guy episode until they recommissioned a fourth series, agreed, it's one of the best programmes ever made.
I own every single box set of 24. It's not worth watching on television because you might miss an episode and that would be devastating because of the continuity aspect.