If you really want your images to look as good as possible you're best bet is to get your own projector and calibrate/profile it for use with your own laptop. Plugging your laptop into an unknown projector will likely result in a display ranging from somewhat acceptable to gawd awful. Of course screens vary as well, so unless you bring your own screen or are able to create a profile for the screen you'll be using you might still get a surprise.
On the resolution side, the smallest common portable projector resolution is 800x600 and most of these will accept images up to about 1280x1024 or larger and scale the image to fit it's native size, no cropping. The most common size portables these days are 1024x768 so I would suggest sizing to 1024x768 for portables.
I'm not sure what the projector will do to your image quality when it scales it down to it's native size? Shouldn't do too much damage but resizing in Photoshop would be a better bet. Images sized smaller than the projectors resolution will appear smaller on the screen with a black border.
If you're going to be using large conference room projectors the native resolution could be as much as 4096x2400 (which your laptops video card probably won't handle anyway) so you would be best to find out what the resolution is and resize your images accordingly especially if you're expecting a large audience.
A good place to find projector specs is http://www.projectorcentral.com
Good luck. Steve.