First off, welcome back TS. Second of all, I feel the base model iMac IS the budget model though it is priced too high. They should have dropped it down to $1,199 if they were going with Iris Pro.
I guess the budget model which maybe will start at $999 will have the successor to Iris, the next model up with succeed Iris Pro, and then it will go 850M, 860M, 875M and 880M. I would think the 880M has 4 GB of memory and everything else has at least 2 GB.