With the future belonging to multiple CPU cores (today, and even better support with Snow Leopard), a dual-core Mac is a dead-end (all models of the Mac Pro have at least 4 CPU cores).
If you’re a photographer, a Mac Pro is the right choice, even if it’s a slightly older model. You’ll have 4 or 8 CPU cores, up to 32GB memory, space for four internal drives and more.
A MacBook Pro or iMac can’t grow with you. One internal drive, expensive memory limited to 8GB, inferior or no expansion options. Don’t get suckered into a dead-end machine unless you have a fixed requirement like the need to travel, or a physical space constraint.
The pity is that a laptop or iMac system can cost more than a robust Mac Pro (total system cost including memory, screen, backup drives, etc). As of this writing, a refurbished Mac Pro Nehalem was $2149. Add a cheap 24" inch screen for $300 or so, and add/upgrade as your budget allows, rather than having to buy another Mac when you hit the limitations of the MacBook Pro or iMac (and waste your time every day along the way).