Not only will the Macbook Air lighten your load, it will do the same for your wallet. No optical drive, no firewire? Sheesh. And if a small HDD is a problem now...I doubt it's 80GB drive will do much for you.

As others have mentioned, methinks the better option is ditching the messenger bag. I used a bag for a few weeks, and as you're finding, you feel every gram. I'm more comfortable with 20lbs strapped to my rear rack than with 5 lbs in a bag.