In my opinion, you don't have the Hardware to show off anything too impressive.

Also I don't know what kind of display you plan on showing this off on. If you plan on using displaying on some large LCD, I'd go for a game like Nexuiz, its generally a darkly themed game, so bloom and lighting effects will be more dramatic. If instead you're looking at a projector or some other large format display where you may not have as much contrast, I'd go with a Warsow demo.