You want to stay away from 'Netbooks and look at Notebooks if you want to run games. Netbooks use low power CPU/GPUs.
Asus 1215n won't do well with SC2 and L4D, Trackmania and CS:S are just 'barely' playable at low settings. Nvidia ION 2 graphics for Atom NetbooksAsus Eee PC 1215N Netbook reviewLenovo G560 15" Notebook $450
Pentium P6200(2.13GHz) CPU 3GB RAM 320GB HDD 15.6" 1366x768 LCD w/ Intel HD Graphics
Intel HD Graphics is still not a 'gaming' solution but it's about the best you'll find under $500.Intel HD Graphics review