what is the bottleneck ?

Hi,My system: P4 2.8ghz, 1gb ram DDR400, radeon 9800pro 128mbFS runs smooth, default Seattle is about 50fps (autogen off), AA 4x, AF 8x, but when lots of scenery is loaded or even panning in default 777 virtual cockpit (front view), I see fast lockups and then graphics is "released" and smoother again.It is worst if I turn water details at MAX, looks like it is something with textures performance, any idea ?Thanks,Ulisses