Have a look at click here.
I paid £1200 for a base unit ( no monitor or keyboard and onboard sound). Spec. is Intel E6700, 4Gb Corsair DDR 800 Nvidia 8800GTX 768Mb graphics card and 750W power supply.
This will run most of the simulators but still struggles to run MS Filght simulator X at a decent frame rate.
Perhaps the most important item for simulators is the monitor so I have the 24" Benq 1900 x 1220 res. which I can highly reccommend.
I can also reccommend Cougar as my system came within three days and is very well biult.