Mac Pro for video editing (Premiere, After Effects)

I'm looking at purchasing a new Mac Pro and even after hours of research I have some unanswered questions.
I will primarily be using this computer as a video editing machine, using Premiere and After Effects. I would like to utilize the CUDA rendering engine and also use Dynamic Link without too much lag (I'm used to any linked files being extremely slow to work with, would love to speed this up). I'm looking to speed up my general workflow, rendering, etc as I find I'm sitting around a lot waiting for things to load or effects to render.

This is currently what I have configured so far...
Two 2.40GHz 6-Core Intel Xeon processors (12 cores)
24GB (6X4GB) memory
1TB 7200-rpm Serial ATA 3Gb/s hard drive (for operating system and applications)
2TB 7200-rpm Serial ATA 3Gb/s hard drive (for storage and working from)
2TB 7200-rpm Serial ATA 3Gb/s hard drive (scratch drive?)
ATI Radeon HD 5870 1GB

My questions are as follows...
- Would I see a big difference in performance from an upgraded (2.66GHz or 3.06GHz) processor over the base 2.4GHz?
- I've done research on the ideal memory slots, setup, etc and it seems 6x4gb is the best option unless I go up to 6x8gb
- I understand the concept of keeping the operating system and applications on a separate drive... but how should I go about storage, working from that drive, should I keep a drive for scratch files/cache?
- What will the performance difference be from the ATI Radeon 5770 vs upgraded 5870?

Any information is appreciated! It's hard to find answers to all these questions in one place it seems... Thanks :)