The build w/ a 6850 is $600, w/ a 6950 $680 and w/ a 6970 $780. All 3 versions of the build are vastly superior in terms of gaming and general use for a gamer / media enthusiast, the OS can be torrented without issues (loaders work without any issues - you won't even know the difference between a legit system and a cracked one).
I wouldn't torrent the main piece of a software in a computer. Yes there might not be any issues but still I wouldn't do it. Torrent anything else you want, but buy a legit copy of windows.