You're looking to do some kind of priority-based networking queueing analysis. Sure, I do this stuff for fun. This wierd old guy across the street, Khosrow Sohraby (we call him Kapitain Krows) taught me all about it.
Actually, aren't there tools to help you do this? I seem to recall something called TK Solver that came out a few years ago and will do this kind of calculation for you. Why do it yourself?
If all else fails,
here is a series of class notes on how to solve this kind of a problem. Covers M/G/1 with variable priority classes and also some other related stuff like G/G/1. If you're asking this question, the math should be comprehensable.
- G