Get the 2.66
Spend the extra $650+tax for the GPU & faster processor and don't look back. You won't be second guessing yourself that way. You also won't be replacing your processor or graphics card for a long time, if ever.
I'm all for saving a buck, but in the long run, it's better to spend, use, and get more for your money. The same logic you're using in buying a Mac Pro instead of a Mac Mini is the same logic I'm applying toward the 2.66 processor & the X1900.
If you really don't know what to do, just get the processor. $300 for 1/3 more GHz from the 2.0 is such a no brainer. You will get faster results almost every time you use your computer.
Consider the jump in processing speed of .16GHz, 20GB more storage, 128MB more RAM in the GPU, and an extra 512 of RAM in the Macbook Pro for $500. Upgrading the 2.0's RAM is $100, upgrading the hard drive is $100, leaving the slight GPU bump (basically the same card) in RAM and the .16GHz speed bump at around $300.
You're getting 4x the speed bump for almost the same price. Do you really think that 512 more RAM is worth $100? Do you really think that 20 extra GB of hard drive space is worth $100?
The 2.66 is the best deal by far of the 3 offered CPU levels.
Just my humble opinion.
(FWIW, I hate the GPU in my Dual 1.8. I wish I would've spent more and gotten a decent card to start with. I do not wish to spend any more money on this computer upgrading it. I wish I would have spent more up front, so I could benefit from the money spent throughout the life cycle of the computer. Now, I'd be spending only 1/2 as much on a better GPU, but given the 2 years or so left I'd be keeping this computer, I don't believe it would be a prudent purchase.) GeForce FX 5200 for reference. I've wanted a 30" for a long time now.
Remorse, remorse... Don't make the same mistake I did...
Oh, and I 2nd the Dell. I have a 20" I bought for almost $900 back in the day. Seamless, reliable, and I like having a black edge bordering my screen more than a silver one. It "frames" what is displayed on the screen better for me, differentiating what is on the screen from my living room behind it (huge jump in visual depth from the edge of my screen to the tv/wall 11-12 feet behind it). Yes, my workstation is FACING toward my living room. Allows me to watch tv, monitor the kid, socialize with people while still being online at my desktop (usually with people standing over my shoulder because we're excited about Googling or Wikiing some trivial piece of information).
Sorry, off topic. My apologies. Good luck with your Mac Pro.