For what it is worth, here's what I found recently. I did this because of things i read that said SP2 fixed a lot of framerate problems. You might want to try....

I'm running an XP machine, AMD 64 bit 3800 (2 GHZ) with 2 gig RAM.
NVIDIA GeForce 8500 with 256meg video memory.

I did a clean install of FSX, and loaded FSX SP-1, which is what you have. Installed the FSX C-130 and had about 8 to 10 framerate.

I then installed FSX SP-2, and the framerate number doubled. I run a pretty consistent 20 FPS, with sliders at about 60% of max range (medium high).

Not sure if SP-2 would help you, but I noticed a difference.