What fps do you get with those specs, i have recently bought a 9800gtx and fs2004 is running superb, i get an average of 40 fps on the ground at San Francisco and highest 60. THe one thing that is weird is that the PMDS for fsx gets 15 fps, i have all setting s very low, traffic none. I want the pmdg do bad, does any one know the difference between the level-d and pmdg 747 in fps.
Thanks,
Zach Beauvais (future pilot) 
Well, the FPS will vary a bit depending on the weather as i'm using Active sky V6.5, but i rarely see a drop below 25-28 FPS at KJFK with 100% AI aircraft and routes (and i'm starting to get alot of WOAI airline packs installed now) and in bad, cloudy weather with the PMDG 747-400.
As for the performance difference between the PMDG 747-400 and the Level D 767; all i can say is that i can only compare the two planes in FS9 as that's the sim i'm using for airliners and from what i've seen the Level D 767 uses significantly less computer resources than the PMDG 747, but then again the 747 is a bit more advanced, with more features and goodies.
However, the Level D 767 is still one of the best airliner addons ever made for flightsim and i bet it will remain so for the forseeable future, despite it's age
