Looks like your considering the 27 inch iMac? I'd put my money into memory and/or flash storage. Either GPU will work fine with Aperture.
Remember if you are getting the 27in that is one of the last Mac's you can still add memory to yourself so go for the minimum from Apple (over priced) and the extra memory once you get it.
It seems clear from the chatter on several Apple-related forums that the Aperture software takes advantage of the GPU, not the CPU.
I'm pretty sure the OS and the app are well-enough engineered to use whatever is available.
... if I buy a new iMac, should I pay the extra $200 to get the 780m GPU with 4mb of video RAM?
Or will the 775m GPU with 2Mb of video RAM do an adequte job?
This is a misleading syllogism. The default GPU is adequate. You might still pay $200 more for to upgrade it.
In my experience, it's always worth bumping up the initial configuration as a hedge against early obsolescence. $200 spent now might give you 8 months more use from the machine in three years. By all means put at least 16 GB of RAM in it.
I do think Retina-resolution displays are an unstoppable force, and that anyone working in photography is going to need one sooner rather than later.