My primary machine is a 2008 iMac (specs in sig). Aperture works on it fine, but it consumes all 6GB of RAM frequently, especially when zooming in 100% and brushing in sharpening or brushing in the repair tool. As such, it pages out every so often which is not really TOO noticeable as I am using a home-grown FusionDrive, but it does beachball every so often.
When I am on the go, I've run Aperture on both a 2012 i7 MBA and a 2013 i7 MBA (both with 8GB of RAM); it is MUCH faster on either Air than on my desktop. I see minimal delays when applying brushes, zooming to 100% or using the repair tool. No beachballing at all and just a pleasure to use. I am shooting Nikon D300 RAW files (10MB - 12MB each).
When my 2008 iMac gives up the ghost, I was planning on replacing it with a high-end Mac Mini; now I'm leaning towards an Air as the experience has been great.