Tallyho, exactly what games do you want to play. What kind of setup do you want? Low power? Small form factor? Or just best bang for the buck?
I say if you are interested in any games at all after 2008, you will want a discrete graphics card... and the atom systems suck for expansion. Even if you buy a board with 1 PCIe connector and plug in a killer video card, the CPU will limit your FPS in the game so much... Atom is mostly made for ultra low power servers or HTPC systems, but can play some games at the low settings.
The best deal on the market for something new, decent and cheap would be to find a LGA775 motherboard with PCIe 2.0, and get a Pentium E6500 or the like (it is basically a core 2 duo e7500 with less cache, then invest in a decent graphics card... the video card doesn't need to be over $100 to play most games... even buying something like a 9800GT for $60-90 would be killer.
Give us more input on your budget and exact uses for the system, then it will be much easier to select the right system.