It has always had 8 GB. It was great with Snow Leopard, but Lion just seemed to be so sluggish, not to mention so many early problems with WiFi, especially, crashing the machine. Although, my MacBook (mid 2011) never exhibited either of these problems.
But I tell no lies when I say that after installing Mountain Lion, my iMac really feels like a new machine again. It's snappy and responsive again.
After my experience with Lion, I vowed NEVER to upgrade an OS on day one of it's release EVER again. But I couldn't help myself
And on this occasion I'm glad for that!