I use 1024x768 (iPad2 res.) and i think that's fine. If your game is going to be very detailed in sprites etc., then i will recommend doing 2048x1536.
And if the game is for PC, then it.. actually doesn't really matter, but in my opinion the minimum is 1024x768.
1920x1080 is also a good resolution, i think.
I think it depends on what platform your game is for :)