I recently suffered through months of debating a similar choice.
I couldn't live with myself if I dropped $3000 on a Macbook with glued in batteries and soldered RAM, so I went about looking at other options. Thought about Mac Mini, but to me expandability is easier than an iMac, but not something I'm really keen on doing, or comfortable performing.
I started reading MacPerformaceGuide.com, and patiently waited for a 2010 6 core 3.33 ghz Mac Pro to surface in the local used ads based on the recommendations on that site and on the site Rajan posted
I found one with 26gb RAM and a 1 TB hard drive. I bought a 128gb SSD for the boot drive, bought another 1 TB hard drive (currently debating to RAID 0 or not), and will eventually max the RAM.
I work with 24mb RAW files, sometimes stitching 10+ images and it performs well for me.
I can sleep at night knowing in 3-4 years I can add another processor from OWC, expand the internal memory, add a PCIe card with integrated SSD, more SSDs and have a relatively fast computer for the next 5-7 years.
I suggest spending a few evenings readings reading the sites mentioned. I can say that they helped me a great deal.
Good luck!