I have quadra 4000, gtx 560, and the new 690 ... I haven't noticed any speed difference in transcoding ... so I'd go with the 570 unless your doing a lot of 3d graphics/raytracing compositing.
Make sure you get the board that has external esata ports, esata ROCKS. I run 24 tbytes of esata on gspeed (in two 12 terabyte chunks). Seems to me you always need more disk then you'd think.
I am moving from 32 gig of ram to 48 gig on my newer machines(12x 3 gig of ram is required for fully using after effects at 4k).
For my new machines i dropped from the 3960x to to the 3930k (the critical thing of the 3930 is the 4x bus , which is better then the 3770 ... but 3770 is a great price point [higher end bus is needed for the gtx690 though]).
Cory Petkovsek mentions your bottle neck will be with i/o, I 100% agree ... for the under $2.5k I'd downgrade to 3770 and have higher end disk and ram, then have low end i/o and fast cpu - depends on your price point though.
No matter what, your not going to transcode r3d @1:1 realtime without a red rocket board ( I normally run at 1/4 res on r3d fine in all my setups).