Based on my experience, I doubt you'd need more than 2 GB of RAM for graphic design work (though more is pretty much always better). Especially since you said that cost is an option, just go with the 2 GB and see how that works out at first, then go from there. You can always upgrade later.
I have a dual-processor PowerMac G5 at my office (I'm a researcher working with 8 GB text data files) and only once in a while, when analyzing these files, do I ever go near the 4 GB of RAM installed in the machine. My iMac at home has 2 GB of RAM and it's more than enough for just about everything. I rarely hit the swap space. Mac OS X likes about 750 MB of RAM for a "happy" system, which would leave about 1.25 GB of RAM for your graphic design work. Unless you're working on absolutely massive graphics, you should be able to fit most stuff into that memory.
But anyway, RAM is so expensive that I'd just go with the 2 GB at first and if you notice some heavy swap use, then up it to 4 GB. If you can, get 2 GB sticks (they're more expensive, but will allow more room to grow in the future.)
Hope this helps.