Thank you to both of you. This is very helpful.
So if I want to upgrade from the base model, I should spend on increasing the number of processors and not worry about the higher graphics cards or increasing memory.
When I look at my iMac now when doing renderings, it only seems to be using 6.5 gb of memory (1.25 wired, 5.25 active) leaving the rest available (5.25 inactive, 0 free). So that seems to corroborate the idea that buying more memory will not increase rendering times.
It looks like ship times have already pushed out to February, so I guess I won't be getting a new computer any time soon!