RCC,
May I make a suggestion? Don't buy the pressure washer. Instead get one of those hand-pump insect sprayers (2 gallon or so) and fill with hot water, 1 cup liquid spic and span, and 1-1 1/2 cups chlorine bleach. Spray on your siding starting from the bottom up.

It doesn't affect the color of your siding, and it kills any mold spores and washes off the dirt.

It's an incredible way to clean your house on the cheap. And sometimes it can save you from having to paint your house so quickly.

I've personally used this method for 18 years on our house that is solid stain on clapboard. It's amazing.