After considering everything, I've decided to just stick with the tent I have now and spend the money I would've spent on a bivy on better clothes and other crap.

I think my tent weighs in at 3lbs10oz, but it has enough room for me and my pack. It has serious condensation problems, and they're not the best for wet weather, but I've been using the same model for about 3 years now and am pretty familiar with them.

I was in Juneau for two weeks a few years ago doing volunteer trail maintenance, and the mosquitoes and black flies were ridiculous. And since I really don't like being bitten by insects, I think I'll take the bug protection of my tent over a bivy too. Thanks for all the replies.