Looking to purchase Mac Pro (dual CPU) for Aperture/Photoshop

Question

My first question is: Will I be able to upgrade the CPU later on? If I were to get the dual 2.26Ghz setup which is currently the base processor (and, the most affordable option), would I be able to purchase the 2.93Ghz CPUs few years down the line and install them?
I certainly know that memory can be upgraded, but I'd like this machine (which, is in itself a considerable expense) to last for at least six years with upgrades.
I currently use 2.16Ghz Core Duo MBP with 2Gb RAM (maximum it can take) and my Aperture and Photoshop have trouble running simultaneously while editing 12 megapixel files from my camera.
Thanks a million guys!

Answer 1

I came from a MacBook, the very first MacBook ever, also maxed out at 2 GB (shared RAM), so I can completely relate to your frustration.
I'm running PS CS3 and Aperture 2 on the Mac Pro with the dual quads and am quite happy. My XSi also generates 12 MP RAW files. 6 GB of RAM is more than enough to run Aperture and PS simultaneously. I can do things like HDR processing with multiple 16-bit TIFF outputs and process them using masking without quitting Aperture. It's just beautiful how much it can do.
The GT 120 seems to be quite capable of making lots of adjustments in Aperture simultaneously without perceivable lag. For most of my non-studio work, I'm retouching, adjusting wb, colors, levels, enhance, and changing exposure settings all at the same time. The GT 120 seems to keep everything I've been able to throw at it, and I throw a lot. It's even speedy while zoomed in at 100%.
At the Apple store, I played with the single quad-core version and was blown away by it's performance as well. Honestly, I didn't perceive any difference between the dual and the single quad versions as far as performance in Aperture and Photoshop. Of course, with half the RAM, you won't be able to have as many layers. When I used the one at the store, I was so blown away, I was afraid the dual quad version was a waste of money.
I wouldn't count on being able to upgrade the processors later, but my biggest concern is that we won't ever be able to really take advantage of all the power the Mac Pro already offers. When you output from Aperture, it's really not that fast, the machine just spends all it's time waiting for the hard drive anyway. The Mac Pro is a server-grade machine through and through. As a still photographer, I can assure you, you'll be blown away by the power of Nehalem and bummed at your inability to use it anywhere near it's full potential. It'll take YEARS for Intel's consumer-grade processors (like the i7) to catch up with the Xeons, and even then, they probably won't support ECC RAM. And each core can handle two sets of instructions at the same time, so virtually, the dual-quad has 16 cores.
With the amount of focus being placed upon harnessing the power of GPU, the power of the CPU is likely to have less and less impact on the speed of your machine as time passes. If you're worried about the RAM ceiling of the single quad Mac Pro, buy the dual-quad, but I wouldn't worry so much about the clockspeed. You should keep in mind, Aperture can't handle the multiple graphics cards, so I'd get a single GT 120 and consider buying something more substantial later, *after you get Snow Leopard*. Snow Leopard is going to use the GPU in ways we've never seen before and I predict it will be a game-changing release.
I'd also check out the refurbs! Hope this helps!