I can testify that it is absolutely worth it. As far as I am concerned, the 2.66 quad is probably the best value for your dollar of all the 09s. I personally went with the 2.93 and have been very pleased, however that was before I knew that it was a relatively easy job to replace the processors with a cheap i7 3.33 GHz later down the road for about 1/4 the price of what it would cost to replace the processors on the octos (since they need xenon cpus) and with much less work too.
I don't know if this matters to you at all but as an example it runs the crysis demo at the maximum settings (except AA) at a very smooth and playable rate... this is despite the fact that the 4870 only has 512MB of ram, yet the system RAM is fast enough that you would never notice. When I was using 32-bit W7 and 3 gigs of ram (only 2 of which were used) it was unplayable, but with 6GB and the 64-bit version it is amazing the difference you get.
Remember that the max ram is actually 16GB, not the 8 that apple advertises. Trouble is it costs a lot for 4GB sticks right now. So get 6GB now for $150, then in a year or 2 get 12 or 16 for $250-300 by then.