At major part of time, I use 320 x 240 for horizontal games, and 240 x 320 for vertical games. One of few exceptions is PIXEL FIGHTERS, with this game I made an INSANITY: I decided use 160 x 120 (is too small!) and the usable area for player is 125 x 120!!! When I'm going to draw, I must increase zoom at 2000 times, hehehe!
So, DON'T MAKE THE SAME INSANITY
320 X 240 or his multiples are cool.
I suggest you play and observe GYNOUG (MEGA DRIVE) and LEGENDARY WINGS (NES) again, mainly LEGENDARY WINGS, it shows a good transition between vertical and horizontal game style, with good use of the screen.
My tv and PC screen are 4:3, maybe because this I prefer 320 x 240
