I highly recommend you get the iMac 24" and max out the RAM.
The speed is excellent for what you need and you would use external HardDrives anyway, at least one just for backup/mirroring. You can fit a 2nd monitor if required as a pallette monitor which is always a good idea but not absolutely necessary for the 24".
Principally you want the 24"s much larger and brighter screen which is glass so you can wipe it down. The smaller screens have a soft plastic finish which is very hard to clean which is a bummer when you need to do editing.
The only downside with the 24" and other iMacs at the moment is the limited number of ports and their awkward placement on the back. However we shall probably see new models next week and these may have been addressed.
Either way I wouldn't act till all 3 factors are in place:
1. Leopard comes with your Mac
2. iLife and iWorks have been upgraded - even as a professional photographer these are very useful
3. The new models are out for a while and you get a chance to see how well they stack up.
You should be able to also get the iMac from regular Apple resellers.
Here in Australia they are freely available from department stores and lots of electronics resellers. All are doing some kind of finance deals, usually GE Money, which are mostly sucker deals. "Interest Free" but charging a monthly fee which turns out to be the same thing as being charged commercial interest.
The Department stores however have their own finance cards and at least a couple have incentives attached ie a discount on initial purchases and also 12 month real interest free with payment at the end of the 12 months. My son has invested his money and when the 12 months is due he should have got an effective 7-8% discount on top of what he will negotiate when he buys the iMac.
I cannot recommend the iMac highly enough it truely is a professional machine for anyone who doesn't need the absolute max grunt of a MacPro which in IMHO needs a redesign and sharp price adjustment before I'd even remotely be interested.