In that price range, I think that the best is the Acer Aspire One. It’s a bit quicker and better built than the Asus, and those are really the only two under $400 unless there has been a price drop from other makers in the last couple of months.
Unfortunately, neither has the Ion. The only Ion-equipped netbooks I’ve seen in that price range are the HP Mini 311 and Asus Eee 1005, neither of which are particularly good in my opinion (too slow).
Of course, shopping around may net you a better deal, but you’re still not going to get a great graphics powerhouse with either a netbook or for under $400, especially considering that few netbooks even use the Ion. You should read this While the Ion-equipped netbooks have better graphics performance than the average $500 ultra-portable laptop, the weak CPUs and small batteries work against them.
For an extra $100 you might find something with better graphics, longer battery life, three times the RAM, and far more processing power; enough to handle the sort of games that would require a decent GPU.
So, I must ask this; what exactly are you looking for the thing to do? Are you just looking at netbooks for price?