I was wondering just how durable to dirt/dust/moisture SLR cameras are? I'm not talking those $1,500 bodies , but something along the lines of my Elan 7. How much can they take, in terms of mist, light drizzles, & dusty/sandy enviornments, before it ends up effecting the camera? Are there any precautions one could take, or any devices to shield against the damage?

I'm thinking of spending $1,200 on a Canon 300mm L lens, and wasn't sure if I'd be better off just replacing my camera body, and then getting the lens at a later time. Opinion?
I shoot at a lot of beaches, and at some point in the future I plan to visit Europe & Asia, so something reliable would be nice.

All replies welcome.