I find this very common now a days... Quite a lot of people think the 9400M is a "Godly" GPU. Yes it's a step up for the Intel on-board one the macs used to be packed with... But that doesn't mean much.
The 9400M may very well be able to load up both games, but the in-game experience will be like hell. Unless you are accustomed to playing games in "choppy" mode, you are in for a dissappointment.
I could see SC2 being remotely "playable" when its choppy. But D3? Looting and PVP will be very difficult.
The Havoc engine D3 uses looks quite demanding on the GPU, regardless of what Blizzard has done in the past with making games for the masses. SC2 isn't due for at least a year, and D3 isn't due for another 2 years, a 9400M will be obselete by then.
The CPU's in the new iMacs will be able to keep up with the game, but your video card is going to create a bottleneck.