I've got FSX locked on 30 frames/sec and it runs great.
I don't have the graphics on max, but there are many settings to balance and it flows well over major cities.
My system is an Intel 950 chip overclocked to 4.20 GHz
6 GB Ram Mushkin Redline
ASUS P6T Deluxe V2 Motherboard
ATI 5870 video card running 3 24 inch monitors
Noctua D14 Cooler
2 Velociraptor hard drives
Track IR head tracker
FSUICP for controls
Windows 7-64 Bit
Computer is used as a simulator only
I spent about $2400 for all the parts and many hours of time
Currently running Level D 767 along with Active Sky Advanced for weather, REX for textures, UTX for terrain, FS Genesis for mesh
Couldn't be happier with this setup, but I had to build it myself and lots of tweaking to get it to run well. FSX is a very CPU intensive program and it's got to be overclocked along with getting the right type of RAM with certain timing and defragmenting the hard drives
There is some great info available about how to get FSX running well and what parts to get for best performance.
This is the first computer I've built and I invested lots of time, but the results are impressive.
I got all my info from reading through the following forums
http://www.simforums.com/forums/
NickN is very knowledgeable and knows the best setup. I followed all his advice and it worked out great
http://www.simforums.com/forums/forum_posts.asp?TID=34141&PID=198187#198187
http://forums1.avsim.net/
http://www.leveldsim.com/sevensix_home.asp
http://www.overclockers.com/forums/?s=
http://www.forums.pureoverclock.com/index.php