I have heard for what I think is a good source that Aperture 3 is more GPU hungry than memory. I have a Aug 2007 aluminium iMac with 4GB DDR2 SDRAM and a ADI,RadeonHD2600 Chipset.
My iMac has started to beach ball quite a bit recently when running Aperture 3. It may be to do with something else of course as I will have Safari and Mail and Twitter open at the same time.
If , as I hope , I get a new iMac soon what upgrade in the spec of a 27” would you recommend for smooth very fast Aperture 3