I have a cooler master eXtreme Power Plus 550W with a power rate of DC output +3.3v/25A, +5v/25A, +12.1/16A, +12v2/16A, -5v/0.8A, -12V/0.8A 5Vsb/2.5A. i7 920 OC 3.6Ghz gainward 285GTX OC 700Mgz.
I find that i get slow FPS in FSX about 15fps over new york city maxed out. Yet if i run 7800GTX 256meg x2 Sli im getting better FPS using 182.06 drivers vista 64 6Gb ram.
The 7800GTX cards have given me way better FPS no matter were i fly. I have run the 285 on a 2nd PSU with 1 and 2 power connectors with no change in performance
It is posible.
EVGA says that their GTX285 requires a 550 watt psu with 40amps on the 12v rails.
You are ok on the 550watts, but your total amps is 32 at most, and the maxumum combined amps is usually less than the sum of the individual rails. If a vga card does not get enough power, you will get strange performance, or it will revert to 2d mode. Still, vga vendors are overly conservative on what works, so you may still be ok.
I think there is an option in FSX to run using DX10 which will improve performance. I think it is a FSX patch and an option setting.