Hi,
Are you playing back h264 footage? In my experience this doesn't work to well, you'd be better with a frame based codec like ProRes or an image sequence such as DPX or TIFF, this should make a huge difference.
MacPro is a good shout if you are looking seriously at grading, however bare in mind that you will need ultra fast storage for RT playback, RAID-ed SAS or SATA storage. Maybe hang on for apple to do something with the MP as the logic boracd on the current 12c model is essentially the same as 2009/10 releases and starting to show it's age.
I have no experience with GT X 570 cards, The Nvidia Quadro 4000 is officially recommended, we use these in outr MP's and they work really well.
Doug