For most games, the HD 4000 should be faster than the 320M. As it is, many games are about the same on the HD 3000 as the 320M. The HD 4000 is supposed to be about 30-60% faster than the HD 3000 from what I have read.
That sort of defeats the purpose of using a thin notebook. These things happen. How bad is the dent? I had a small crease in my 2008 MacBook Air, and it affected the resale value by about 15% when I traded it in at PowerMax. As long as it still closes properly and the screen itself isn't damaged, my guess is that you are talking a reduction in value of $100 tops. Someone buying a 2010 MacBook Air in mid-2012 is looking for something inexpensive, anyway.