I've actually done something pretty similar recently. I was fed up with Apple's pricing strategy on their Mac Pro line and decided to build myself an i7 hackintosh - well, in a way I upgraded my Mac Pro.
Specs before upgrade:
2.66Ghz Dual Dual Core Intel Xeon (Woodcrest 5150)
6GB 667Mhz DDR2 ECC RAM (Dual Channel)
2 x ATI Radeon 4870 1GB XFX cards (flashed - also limited to 8xPCIe1.1)
16x Superdrive
750GB System and User data drive
500GB Backup data drive
3x1.5TB Data drives
Bluetooth + Airport Extreme
Specs after upgrade:
2.66GHz Quad Core Intel Core i7 (920 D0) running at 4.2GHz
12GB 1600MHz DDR3 RAM (Tripple Channel)
2 x ATI Radeon 4870 1GB XFX cards (now using 16xPCIe2.0)
8x LG Bluray Reader/DVD Writer (meant to order the BD-writer, whoops)
256GB Crucial M225 SSD System drive
500GB User data drive
750GB Backup data drive
3x1.5TB Data drives
Belkin Bluetooth
XBench before: 156.06
XBench after: 410.47
Even better, the simulations I have to run for work now run over twice as fast and three times as fast if I'm running eight of them thanks to Hyperthreading.
Total cost of upgrading was about £1500 although I should recoup a fair chunk of that by selling off the old Mac Pro parts. It's running incredibly well at the moment, OS X is great, Windows is great. The only issue I have is that I can't get my second 4870 to work in OS X no matter what I try. Half the time I start up, its fans are spinning like mad until I restart. I only have the second 4870 for Windows crossfire use anyway but I'd like it to work in OS X for CUDA maybe and to stop the fan from being noisy. I'll probably upgrade to a 5970 when 5xxx series drivers get added into OS X though.