I just wanted to add my thoughts for this. I just read in either one of the links posted in this thread or another thread that the radeon hd 6630m is supposedly? equivalent to the desktop radeon 5570. The 6630m is based on 5xxx series architecture and has 480 steam processors. The 256gb ram it has on it is gddr5 ram, not gddr3, so that would make a difference since its pretty much 2x as quick.
However, 256mb isn't a whole lot of ram and even though its faster, I think more would be needed to play higher resolutions and game.
So, if I was guessing, well I know that the 9400m is alot worse, and you would probably be able to play portal 2 and diablo 3 at low-medium settings, depending on how big your resolution is. That would gobble up the v-ram.