Got a Quad 2.66 demo in May, bumped the ram to 7GB and lllloooooove it! Main reason was my hot-rodded G4 dual 1.8 (Sonnet CPU upgrade)was starting to power off when it ran too hot, esp. when working on a 95-minute-35-track film project (Logic 7.2). I have yet to peak out the CPU, but then again I haven't thrown convolution reverbs on each and every track.
The new computer has been a gorgeous rock-solid dream come-true for me in terms of ease-of-use and reliability. I've used it to render full-res video files out of Final Cut Pro, burn DVDs in Studio Pro, and best of all it has no problem keeping up with the film project in Logic Pro 8 which has even more audio content now. Plus, it is EXCEPTIONALLY quiet.
The reason I went with the 2.66 instead of the 3.0 was that I found out that the improvements in non-CPU speed areas (like SATA drives vs IDE, bus and cache, RAM 'bandwidth') were a big part of the diff between my old Quicksilver mac and the MacIntels. Of course the Intel CPU structure is important too. BUT some tests (www.xlr8yourmac.com or macworld.com, can't remember) were done showing that the difference in speed between the quad 2.66 and quad 3.0 were 10-11% AT BEST with software that was specificially pushing the the CPUs. Otherwise the speed bump was maybe 5% on average.
If Apple varied the bus speeds and caches in addition to CPU speeds in the Mac Pros that would be a different thing altogether. But to vary ONLY the CPU speed by 10% between the same core count does not really justify the extra $$ for many folks.
I got the Mac Pro after they had been out a while so the bugs could get worked out of the hardware. I suggest that you do the same with the new 45nm based CPUs. Don't pay to be a beta tester for Mac. If you can wait until the new ones have been out for at least 3 months, preferably longer, do so. Then scrounge online for problems people have been having with them so you know what to possibly expect.
You might be better off getting a new quad and sinking the extra $$ into RAM.
Hopefully there won't be any problems at all.
Sorry for the lengthy monologue, but I hope all this was helpful. The Mac Pro is my 5th mac, 3rd tower. My first was a 1992 iivx model with (230MB HD).
Cheers and good luck,
Elizabeth
aka Annadyne, Aurascene