I'm not sure what you're trying to achieve, why pick the i7 extreme with an expensive mobo? You can easily get 80%+ of the power for less than half the cost.
Have you considered putting together a render farm? A power in numbers type of thing. An i7 extreme @ $970 can beat a $100 AMD quad, but I doubt it could beat 5 $100 AMD quads in a render farm. (software licensing may be costly, don't know what you're using)
Ah, well if its a fast workstation you're worried about I personally would go with an i7 920 and OC that a little instead of the 975. Many people in the OC section of the forum have gotten those to 3.5 ~ 4.0 with a stable OC.
I wouldn't buy the best of everything, 2nd or 3rd best is about as good most of the time. Then I'd bank the money I saved on not going all out for upgrades in a couple of years. By then I'd assume that 6 core CPUs would be available for a reasonable price. 8 core CPUs could be released as well.
By all means though, if money is no object, just buy the best.