I have just installed All of PAI's traffic packages, yes every one of them and its fantastic. I'm currently flying a Virgin Atlantic A380 from LA to Heathrow and whilst making my way to take-off the airport was absolutely full of traffic, exactly like it is in real life, i saw almost all of the PAI aircraft I had installed and best of all it hasnt effected my FS2004 gameplay, with all this AI traffic im still getting 30FPS (locked). I also downloaded the flyaway kneeboard which i am well impressed with, i am actually posting this topic whilst still inflight, its amazing, i didnt think FS2004 could get any better!!!
Its worth checking back with the PIA website every so often as they will update various airlines flightplans so that they are true to life(well almost)
Especially now that the summer flights are being published
For a free addon this is one of the best