I'd agree on the aperture priority, however be sure to check over your early shots as soon as you get a chance in case you need to tweak settings. Your ISO will effect the shutter speed (higher ISO = faster shutter speed), but increasing it will increase the noise.
I say you should check your shots early because it can catch you out. My wife and I were shooting some greyhounds running, and we'd discussed settings all week and decided on aperture priority for the reasons suggested by others above: Widest aperture (lowest f number) gives you the fastest shutter speed for your current ISO. If the light changes your shutter speed changes. The problem was that I was set at ISO 400, and my wife was set to 100. She felt disappointed because she got some shots which would have been far better than mine, but were blurry due to a slower shutter speed. If she'd have examined the shots early in camera she could have seen what was happening and upped the ISO before it was too late.
Your other option is to use the AUTO ISO to your advantage. It won't help for aperture priority because it doesn't kick in until much slower shutter speeds (I'm assuming the D50's auto ISO isn't any different to my D80's), but if you wanted you could go with shutter priority and use it.
In shutter priority, you'd pick a decent shutter speed (1/500 or faster), and the aperture will change depending on the light. When you reach your highest aperture, it would start to underexpose the picture, but instead the auto ISO will kick in and up your ISO. This way you can start at ISO 100, and the camera will increase it depending on the light.
DG