FSX Service Pack 1
The FSX Service Pack has a massive performance boost if you are using duo core processors. Before SP1, I had to turn most graphic down, such as autogen, etc - with SP1 I can run FSX with graphics on high and still get my framerates maxing out at 20fps (I've locked it at 20fps). I believe the developers have dedicated one core to pushing graphics to the card, freeing the other to focus on dynamics, etc.
By the way, if you want an inexpensive duo core 2 processor, look for the Intel E2160 which runs at 1.8ghz - it has massive overclocking potential with no additional cooling - you can safely boost it to 3.4ghz (assuming your mobo FSB and RAM can handle it). 'm running it at the stock speed and get great results.