When flying in multi-player, my frame rate is very low (between 5-10, sometimes lower than 5).

I've got a 533 MHZ processor, just under 200 MGB's of RAM, a Raedon 7000 graphics card and I'm on hi-speed internet service.

Where should I start looking?

A couple of quick things to check. If you get good frame rates in single player but not multiplayer, I would check where it is you flying in multiplayer.Some sceneries are like boat anchors to your frame rate.Another item to check is what kind of planes you're flying.Some add on aircraft I discovered will kill frames rates if a lot of people use them. Some versions of the Memphis Belle B-17 will do this.Flies great with one or two planes but 8 of then in the air and even from 50 miles away it will murder your frame rates. I'm not sure what the technical explanation is but it has something to do with the way the object graphics are read.There are certain codes that limit how you "see" a plane.The closer you get,the more detail you get.Some aircraft files don't have this code written into them and they read the "entire" plane no matter where they are in the game.This KILLS frame rates.We had to discard some air craft for this very reason. Hope this helps.