My set up (semi pro photographer, pro graphic designer):
MBP 17", 2.66 core i7, 4gb ram, 500gb 7200rpm ATA drive, Antiglare Screen, 30" ACD, 1tb time capsule, 500gb ext. hard drive.
Main daily software used is CS5 suite and Aperture 3.
To the OP, I couldnt recommend this set up more, its absolutely perfect for what I need and do and I'd say easily powerful for most photography needs.
Both Aperture 3 and P.S CS5 absolutely fly, no problem what-so-ever working on raw files up to 30mg (I shoot with a canon 40D, so around 10 megapixel on highest res) and I cant envisage even needing to upgrade beyond this.
I dont really notice anything RAM wise - can easily run plenty of programs in tandem, and the read/write speeds are plenty fast enough for me. Was considering am SSD from the outset, just to store apps on, but honestly I dont need one! It zooooooms!
Its a fantastic system to take on shoots - the anti glare high res is astonishing and I often use it hooked up to the cam to shoot remotely from (great fun and clients especially love having a peek straight away!) and as a desktop alternative, I cant recommend enough. I'll stick with this for a good few years!
If money was no problem, have a gander at this website, its incredibly informative for mac specific photography set ups (as well as general photography discussion!)
If the computer I own flys, this guys spec. will completely blow it away. would love to try it!
http://mpgprolaptop.com/
Good luck!
P.S - unless you are mainly taking photographs for online use, give all the glossy screens a miss - they will boggle your mind in terms of getting accurate colour for print. Total pain to collaborate and never 100%
Antiglare / matte are your friends!