I did quite a bit of research between the 5xx and 6xx cards before building my new system. I actually, for a short period, had a 7970 (that I eventually returned). I went with the 5xx series for the simple fact the OpenCL performance is better than the 6xx series. When you use CS6 constantly it makes no sense to buy the 6xx series when it is simply geared towards gaming. I have to say though, the 7970 blows the doors off of any consumer grade Nvidia card for OpenCL benching. That card screams. The problem with the 7970 is the driver/OpenGL support on the PC side. It is a mess and software like C4D R13 wouldn't work unless you downgraded ATI drivers 3-4 versions down. As well, for a hackintosh Nvidia is just simply the easiest option. Just did a LuxMark on my 570 (10.8.3) benches at 1144.
Mountain Lion 10.8.3 gives you the best support for the 650 Ti at the present time. Play it safe, don't experiment unless 1) you know what you're doing and 2) make a backup for experimenting. Backup programs such as Carbon Copy Cloner (CCC) and SuperDuper are great ones to use for making backups.