We do not know specifics of how Aperture utilizes discrete graphics cards but in the past solid graphics were a necessity. E.g. Apple's strongest G5 tower would not run Aperture until an upgraded graphics card was installed. Boxes that rely only on integrated graphics like MBAs and Minis are not recommended.
Usually a good general feeling of relative graphics performance can be obtained by diligently perusing the tests at barefeats.com. Note that Aperture-specific testing is IMO only marginally relevant, but overall graphics app performance is IMO very relevant.
I use a 17" i7 MBP with SSD and 16 GB RAM. It works well as a desktop-replacement box, but my RAW files are only 20-30 MB size. My Aperture/Photoshop workflow was paging out at 8 GB RAM before I upgraded to 16 GB RAM. Odds are that 16 GB will work fine with your big files but if I was moving to a MF DSLR I would be a bit concerned about only having 16 GB RAM as the maximum available.
Personally to obtain mobility I would sacrifice the ability to to exceed 16 GB RAM but I would only do it with Apple's strongest mobile GPU available at purchase time. I.e., the top MBP.
If you do choose to give up mobility plan on a 32-GB box when price comparing. Personally I would go with a cylindrical Mac Pro not an iMac because the stock Mac Pro graphics are far stronger and the Mac Pro is upgradable.
All the good choices (MP, top MBP, top iMac) all have more than enough CPU. IMO in 2014 we can mostly ignore CPU as regards purchase decision making on the top boxes.
512 GB SSD for boot is IMO a mandatory minimum for any new Aperture box in 2014. Note also that if you do create a RAM-intensive workflow that does page out 16 GB RAM the deleterious effects of paging out are substantially ameliorated by SSD.
My 02.
-Allen