I can run FSX at completely max detail - 1600x1200, very high settings on everything (extremely dense autogen/scenery), etc, FSAA, anisotropic filtering, etc. and even with complex add-on aircraft it's butter smooth for the most part, even over large cities.
I've got a EVGA 780i SLI FTW mobo, Q9550 (quad core) clocked to 3.83 ghz (FSB @ 1800 mhz), 4 gigs of OCZ DDR2 RAM, an EVGA GTX 280 SSC overclocked a good bit, and a 1000W KingWin power supply.
Can build it now for about $1,400 in parts. It's a monster. The addition of another GTX 280 would make it even more powerful, but I really don't need another one, it runs everything I throw at it just fine.