400MHz isn't much of a difference. It'll save you a bit of time here and there, but you're talking in seconds. If the cost is no issue, then by all means, go for the 2.4GHz and upgrade the RAM later. But if you're looking at the best bang for buck, the 2.0 model is an absolute sweet deal. Adding 2 sticks of 2GB RAM (especially now since it's DDR3) will make the machine absolutely fly. The only other major differences between the 2.0 and 2.4 model is the backlit keyboard, which is kind of a gimmick anyway, and hard drive size. Depends on how important these are to you.
If I was you (as you say, you do a lot of photography/graphics work) I'd be getting the 2.0 aluminium MacBook, upgrading the RAM to 4GB, and putting any extra cash in a top quality external display like an Eizo or some of the NEC offerings. None of the MacBook panels are nearly as good as those in the MacBook Pro. The same goes for the iMac 20" vs the 24", the latter is vastly superior. So any major graphics work would be better done on an external display no matter which model MacBook you get.
Good luck with the purchase. I'm sure you'll be happy no matter which one you buy. The new MacBooks look absolutely stunning.