Go for 2008 model. Its GPU it's a lot better because it's not intergraded. '08 model is cheaper too.
Definitely get an iMac with a discrete GPU, be it last year's model or one of the 2009 ones. I just bought the 2.93 GHz iMac, as it comes with the nVidia 120. I have found it to be very good at fairly demanding games.
Doom 3 runs pretty flawlessly on this hardware.