Im on a hackintosh. I mirror all what Andrew has said. im running an i7 2600 overclocked to 3.8ghz, 16gb of 1600mhz ram, ssd, and a cuda nvida gtx 460 card - the cheapest one which worked with apple. running on Lion. a mate of mine configured it. superb machine. lightening fast.
a few funny things I have found with mine:-
1. Parts of OSX seem to be defaulted to some type of arabic text - which I am unable to work out why. This is not affecting workflow but is a bit annoying. - certain windows such as the cd burner window has all text in arabic. most things are not affected though.
2. The internal sound on the motherboard is twitchy - definitely use an uprated soundcard. using the inbuilt sound on mine means that if you import some audio into premiere and play it back you lose audio on youtube / vimeo due to some conflict somewhere, and you need to reset the machine in order to get audio back. again, not that bad really.
3. When using some of the usb 2/3 ports on some occasions and with certain hard drives, the machine tries to boot from the portable usb hard drive. as a result, when you shut down to restart after disconnecting the usb hd, the system changes the boot setting to ram speed of 1300mhz. If this happens I just need to restart and go into the boot menu and reload the optimised user setting we saved originally so the system is set up to run the ram at the correct 1600mhz.
As far as everything else is concerned, I am over the moon. cost under £900 including Lion and a genuine apple keyboard