well to work with this on photoshop you need a very good machine, not that yours isn't it's just that you seem to need a better one (although i think yours should be enough)... i would recomend you to do it on illustrator that way you could do it small and once you want to print it you can just resize it and as they're vector you would have no fear of it becoming pixelated, that's just an idea of course it also depends on the type of artwork you want to make, if you don't want anything vectored then you could try redirecting some RAM memory to the video... i don't know if it is the same on the macs but on the PCs you can just go to the SETUP and set a new amount of memory for the video that could speed up the process, other than that i have no idea...
NOTE: It is very wired that you're having this problem, cuz i got a PC of 3GBs RAM and a regular video card (NVIDIA GeForce 6150SE nForce 430) and Photoshop works perfectly on that and even bigger canvas... could you tell us the full properties of your mac??? like processor RAM video card?
hope this clears up something
