The drivers you'll need are for the motherboard and the video card, however the motherboard will have several drivers. I did find an HD6850 driver but the comments make it seem very hit-or-miss.
http://www.newegg.com/Product/Product.aspx?Item=N82E16820231416
I would be very surprised if you found a way to get crossfire to work.
A couple of good resources are here:
http://www.kexts.com/index.php
http://wiki.osx86project.org/wiki/index.php/3rdPartyDrivers
http://www.hackintosh.com/
Most Tom's members will simply tell you that OSX is a waste of time and money, but I do think that Apple has a place in the world. I have a homebuilt machine with an i5-2500K for gaming, programming, and ftp, however I'm writing this to you from a macbook pro. That being said, we've surpassed the point where there are things you can only do on a mac or things you can only do on a PC so there is no reason you *need* OSX. From where I'm sitting there are two economical choices:
1) If the price is right, buy your friend's computer and use windows.
2) Buy an iMac. I don't know what your friend is charging you, but that computer was pretty expensive when it was new and by the time you start replacing parts you're probably going to spend more than the price of an iMac and that's nothing to speak of the hassle that it will cause you.
Building a homebuilt computer with OSX is possible, but it is rarely the best economic choice or the best choice for utility. People who do it usually do it for the challenge, because it's fun, or so that they can say that they did. You're in for a long, uphill battle where you'll be editing drivers by hand, spending a lot of money, and spending a lot of time in the far corners of some questionable forums. Not to say that you shouldn't do it, but it's a project so be aware of what you're getting in to.
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