Well, the single 1 GHz machine is certainly a good machine to start with the Macintosh. The things you talk about won't make so much use of the dual processor setup. Photoshop, but only in some cases.
If you ever start to really push the processor(s) with things optimised for dual processors (3D animation, large scale graphics stuff etc.) and you're going to make money off of these features: Why not buy a decent machine for those tasks, then, I ask.
Btw., I AM a graphics designer and am totally stunned at how well I'm doing on my tiny iBook with one G3 processor. Thing is, Mac OS X is helping me so much in these tasks that I don't mind if something in Photoshop or InDesign might take a bit longer sometimes.
However, if you PLAN on keeping this for two years already, the 500 USD don't sound so bad, either. Just keep in mind that everything BESIDES the processor(s) is upgradeable to the same extent. And overtime, a new graphics card or more RAM, a new harddrive or optical drive might seem a good idea. All those things can be upgraded as easily on a single OR dual processor machine.