I assume the amp you will be using will be mounted inside with the airport, just the speakers will be outside right?
With your application in mind, I see no reason for a full size cumbersome amplifier. You just need a simple audio in and some amplified output and a volume knob...http://www.parts-express.com/pe/showdetl.cfm?Partnumber=300-383
That amp will provide more clean power than most full "size" integrated/receiver style units costing twice as much. It's a nice convenient little amp that is perfect for connecting an ipod or airport to to drive some outdoors speakers with. In fact, that's what most people who buy that amp use it for.
Don't buy your speakers or your amplifier based on Watts. Wattage ratings are, far less important than you have been lead to believe. And I already know you have been lead to believe they are important because you are quoting the "wattage" rating of some speakers. Considering the fact that the vast majority of listening takes place at around a watt per channel or less, you can comfortably forget about wattage ratings unless you are planning on having a rock concert in your back yard, in which case, you need very different equipment anyways.
As far as the speakers themselves go, I'm not sure what size/cost range you had in mind, but something like this: [ame="http://www.amazon.com/OSD-Audio-AP840-Performance-White/dp/B002BLWX2E"]Amazon.com
: OSD Audio AP840 Outdoor High Performance Patio Speaker Pair (White): Electronics[/ame] would probably be pretty nice. My suggestion here, since it's for outdoor use where there is no room loading, is to look for outdoor speakers with larger diameter woofers. They'll give you better efficiency down low, and be able to project the lower frequencies better in open-air conditions. I'd say 8" woofers would be about the minimum I would want personally.