Mac Pros are overkill for games, and very expensive. They're workstations, not personal computers. They are due for an update, so definitely by the end of the year at the latest. If you were to get one, get the lowest CPU + the best GPU you can.
iMacs are better than they used to be for games, but the larger resolution the 27" will be a hindrance for running games at max settings, especially in the future. ME3 and AC3 would likely be fine, but don't expect something like The Witcher 2 to run amazingly.
My advice would be to build a gaming PC yourself for ~$800 which will smoke the iMac, and use the rest to buy a Mac Mini or low-end MBP. Best of both worlds.