One quick thing

[Kosheh]

Based on a ton of things which I play, I'd say 640x480. I'd probably go 800x600 if you're feeling most risky, but yeah - once again I'd say that's generally the windowed size. Fullscreen is different and might be like 1280x800 by now, lol

You might also want to go with a widescreen ratio too though, if you're wanting to appeal to people who play fullscreen on their wide monitors. :V