X800XL/PSU Issue

I recently reinstalled XP on my desktop only to find that my FPS in Flight Simulator 2004 were reduced by half. I reinstalled XP, FS9, defragmented, downloaded newest drivers, updates, etc, but nothing worked. I am convinced that my problem is not a software issue.

In Flight Simulator 2004, whenever I moved my view to complex sceneries and buildings the frames took a dive. I got typical FPS when looking directly at the ground or very simple, sparse scenery. This clue hinted me that my X800XL was fading on me. Here's where I need help...

I have a feeling that my PSU (400W, 2 +12 rails at 14 and 15V) is not able to supply my X800XL and Arctic Cooler. It is about 1 year and 2 months old. A feasible next step would be to replace my PSU with a single rail PSU and see what happens. My questions...

1). Will replacing the PSU fix my problems or is my X800XL fried?
2). Does it make sense that my FPS are low because of a graphics related issue?
3.) Any suggestions or input would be appreciated.

Jeff

system:
P4 3.4Ghz
2GB Corsair RAM
80GB Seagate HD
ATI X800XL