PSU should meet the demands, but other drives/cards also take power.
Cooling is for the CPU which is separate from the GPUs. If you run the CPU at 100% for long periods it will still get warm even with the best of cooling.
Graphics cards in SLI provide quicker changes especially if playing intensive games' doing a lot of video editing or similar. They also have their own dedicated RAM for this.
MSFS 10 doesn't need to 'recognise' the graphics cards it is the system that directs loading. Whether you will accomplish using all 12GB RAM will depend on whether the OS can see it (XP would not) and the loading you have on the PC with the software run.