I think you can buy a radiator flush at most FLAPS...probably doesn't work as well as having it rodded out, but might be cheaper, and if it's not that clogged up it might be all you need.
I'm with Achipmunk on the pressure washer....be VERY careful. I would think that soap & water with a brush that has long soft bristles would get the worst of it.
I think Home Depot has a product that HVAC guy use to clean A/C condensor coils, not sure how well it works...never tried it, I'm pretty sure that it contains Phosphoric acid though, so you might want to do some research.
Hope this helps
