The HD3000 GPU has had a lot of unjustified bad rap around here (before it was released).
The order of GPU performance for the GPUs you mention goes something like this:
9400m -> -> (i.e., significant gap) -> -> HD3000 -> 320m
Both the newer GPUs are significantly faster than your 9400m.
The HD3000 and 320m are pretty close. In some things the 320m is marginally faster, but its very very close. The HD3000 is also faster on a few things as well. I think the HD3000 got a bad rep because it wasn't much of an improvement, not because it's totally crap.
However, sandy bridge core I series absolutely DESTROYS the Core2 Duo in the 2010 Airs. Its not even close. Hence, any game or application that is any way CPU bound will be much faster on a 2011.
Especially so in the 11" models, where the fastest 2010 CPU is a 1.6 Core2 Duo.
To give you an idea of just how much faster the new CPUs are - a Core i3 (dual core) has comparable performance to a Core 2 Quad at the same clock speed. You can spec the airs with a Core i7, and the i5s are standard.
A friend had a 2011 13" air for a bit (his first mac, before stepping up to a 15" pro for more RAM for Vmware) and he could run Half Life 2 and a few other common LAN shooters just fine on it. He didn't upgrade because of lack of performance - he really loved the air, just needed more RAM than 4gig. The Air was just a bit of a long term test to see if he could live with a Mac rather than Windows.
Obviously you won't be running the very latest stuff in full detail on it, but for games a couple of years old it should be fine on lower detail settings.
edit:
bear in mind - whichever laptop you buy, running games on it WILL make the fans spin up and make a fair amount of fan noise. My 15" pro can run pretty high end stuff just fine with the AMD 6750 GPU in it, but the noise is quite loud