That actually depends on what I'm doing. No matter what I'm doing, unless I'm only testing a level that I'm building, I play in full-screen mode.

If I'm playing single player (which for me is most of the time), I have the resolution on 512x384. This lets me see enough of the level at one time that I can see as far as I can shoot. I've never had problems with not being able to see far enough in single player on this setting. The added benefit is that because the scenery and the sprites are larger in proportion to the screen, they move fadter across it. Playing at a slightly lower resolution than the maximum width actually increses the sensation of speed when you run! I'm talking actual speed relative to the player's range of vision, not a boost in performance. That makes the experience more intense for me, and more like the original Jazz Jackrabbit.

If I am playing multiplayer, no matter whether it is split-screen local cooperative play or just me on my computer going online against opponents, I play at 640x480 like just about everyone else, because I need as much as possible to be crammed onto the screen in either case.

I really do recommend trying 512x384 for single player play. You'd be might be surprised at the extra rush of speed you can get