Poor FPS in FS9 & FSX. Any reason why?

Hi, I have recently put a pc together for flying my sim, was using a laptop before but without any dedicated graphics memory so it struggled a bit! Anyway, built this machine, the specs are:
2.75mhz dual core intel
3gig of ram
Nvidia Geforce 8800 gtx 768mb
22"hanns g tft
Now I'd have thought that this would have run fs9 superbly on high settings and fsx on medium high but fsx isnt even playable and fs9 seems to average 8-20fps running 1680X1050X32 with, admittedly everything turned up high, and some scenery add ons (vfr scenery, uk200 airports) and active sky v6. But it suns poo, especially in cloud or near traffic. It can run quite well with weather set to clear skies but thats missing the point, I would have thought that this machine would handle it seeing as lesser setups manage ok.
I have done some searching, I asked on another forum and was directed here. I have done the tweaks in the fs9.cfg file and have installed nhancer. These helped but not much.
Its worth noting that when I installed fs9 I was running a 7300gs 256mb and I upgraded to the gtx, could this be an issue? fs9 seems to recognise the new card ok
Any advice would be handy, this question probably gets asked all the time!
Cheers!
Paul
2.75mhz dual core intel
3gig of ram
Nvidia Geforce 8800 gtx 768mb
22"hanns g tft
Now I'd have thought that this would have run fs9 superbly on high settings and fsx on medium high but fsx isnt even playable and fs9 seems to average 8-20fps running 1680X1050X32 with, admittedly everything turned up high, and some scenery add ons (vfr scenery, uk200 airports) and active sky v6. But it suns poo, especially in cloud or near traffic. It can run quite well with weather set to clear skies but thats missing the point, I would have thought that this machine would handle it seeing as lesser setups manage ok.
I have done some searching, I asked on another forum and was directed here. I have done the tweaks in the fs9.cfg file and have installed nhancer. These helped but not much.
Its worth noting that when I installed fs9 I was running a 7300gs 256mb and I upgraded to the gtx, could this be an issue? fs9 seems to recognise the new card ok
Any advice would be handy, this question probably gets asked all the time!
Cheers!
Paul