Not sure if it helps, but I recently bought a 27" Retina iMac with Fusion drive, 4 GHz i7, and 32 GB RAM (get this elsewhere, don't pay the Apple RAM tax).
Way more power than I'd ever use, and the retina display spoils you rotten. It's painful going back to a normal screen now.
Super super quick in all areas. If it's anything like my old machines it should last 5+ years and still be going strong.
As Ted says, you're going to be in for a shock when you realise how quick the new machines are
If you just want to upgrade your old banger then a SSD and more RAM will sort you out.