Yep, this one. Only that I use a flashed one from MVC (
http://www.ebay.ch/itm/330771925657?ssPageName=STRK:MEWNX:IT&_trksid=p3984.m1497.l2649)
I use 10.8, but I'm not sure whether it will work out of the box (if you buy a PC one, with a MVC one 10.7.4 is just fine) in 10.7.4. I know it does with 10.8.
Do you need the GT120? I also have a secondary screen and plugged it directly into the GTX570. That saves me a PCIe slot and the GPU runs with a bit lower temperature.
To be honest, in every day excercise the GT120 did even better than the GTX570. Photoshop did slightley better for example. But as soon as you go into heavy GPU stuff such as gaming or editing, the GTX570 will blow away everything! The GTX570 from is the fastest card you can get full working under Mac OS X (GTX6xx are running PCIe 1.1 speed, so do the stock GTX5xx).
If you can afford the almost 600 bucks, go for it. You will have lots of fun with it.
I would sell the 5870 and stick with a GT120 or even a 8800GT just in case the GTX570 gives up.
No, missing driver support. No chance there. Apple decided to Nvidia, all the new MBPs have Nvidias. This makes it possible to install certain PC GPUs in the MP.