I am having this debate as well. I edit JVC HD 100 footage so a Quad is more than enough, but the question is how long will it be amazing.
I can get a quad with 4 gigs of ram, a 23" inch apple monitor, about 3 hard drives, the ATI Radeon card or a 8-core with a 22" inch Dell, 2 gigs of Ram, the standard hard drive and the ATI Radeon Card.
The idea is that the 8-Core system will be easily upgradeable in regards to Ram and hard drive space, and I can get a better monitor down the line.
Then again, I think the Quad's (2.66 and 3.0) are more than powerful, so why not just get the whole system soon.
Either way, I am waiting until Apple ugrades the machines. The Macs are LONG OVERDUE to be upgraded. It may not be until WWDC in June though.
Now I'm leaning toward the quad for this reason: I think back to a couple of years ago...not that long in the vast sweep of history. I chose the G5 1.8 DP instead of the 2.0 DP. There have been a few generations of Mac towers since then.
Would spending the extra bucks for the 2.0 back then make the slightest difference now, in terms of deciding whether to go for a Mac Pro? No.
And remember when Apple was advertising either the G4 or G5 as a "supercomputer"? I mean, what could be more powerful than that, right?
Moral of the story, kids: It's all the same if you wait long enough. Quad core, 8 core -- before you know it, they'll both be quaint.