I'm waiting for the time when SSDs will be cheap enough, and they'll be the size of a CF card and you'll just pop it into a slot next to the RAM. Will that ever happen?
Or you could just throw a 64 GB SDXC card into the SD slot that's already in the iMac..... (Of course, the SD slot in all Macs are only 480 Mbps USB, but hopefully they'll move to a faster interface in the next iteration.)