The best way from my experience is to upgrade your memory. My iMac has almost the exact configuration as yours, except that I recently upgraded from 8GB to 32GB. The result is much more impressive than I could have imagined - navigating in Aperture is now instant. It seems as if the entire database is cached.

I have also tested Aperture on an SSD-based Mac, but to my surprise I found that adding more memory seems to have an even greater impact (and is more cost effective as well).