Hey Alice, welcome to the forums.
I ll stick to some basics that I am sure most people here will agree with.
A. There's a wide-held view that the best time to buy, is when you really need to irregardless of updates and so on, I am sure a lot of people here can vouch for how much you can get stack if you are expecting the next best thing, there's always a next best thing coming along.
B. That said, it's very reasonable to want to be up to date and not buy and have an update the next day, although apple offers an exchange for any update happening within 14 days of purchase. It's also very reasonable to wait for something that fits your need if the current scheme does not.
C. As for the air in particular: You wont see much difference in the 1.6 and 1.8 airs for cpu speed per se, what is a huge difference is the ssd, so if you want to save some money, go for the bto option of ssd and 1.6. The hd one is still a good machine, but people who 've used the ssd swear by it. There's a rumor that the 1.8 screens get the better specs and dont suffer from the rather widespread issue of tiny lines in the white background affecting the air. But that's a matter of speculation and you can always exchange it or return it if you are not happy. If you don't like sluggishness you ll love the ssd btw. Judging by your needs also it seems 128 gbs are more than enough.
D. As for the updates, well, that's a tricky issue, hence this site, hence the forums. The cpus from intel are already out so apple could potentially update them at any moment, but what has traditionaly been holding back new air releases has been the ssd sizes, and we have yet to see one that will fit the air and be bigger than the current one. So it's fair to assume that apple might wait for it to include it in the update, with the rumored 3g addition which will be great for joining a mobile network much like the iphone, of course if you have the iphone soon tethering (using the iphone as router to connect the air) might be an option. That said apple can just bump the speed a bit and go for a major update much later in early 2010 with the new intel cpus out. I am on the same boat with you since I want an air, but as is things are uncertain, there hasn't been a big product cycle or a fixed one like the ipods and iphones to go by either....
I would wait as much as could, but not a day more, and if within the coming month or so you see no speed bumps, buy it and enjoy it, I am sure you will love the air with or without 3g or a larger ssd anyway. And in anycase the speed bumps wont be anything major or have much real life difference in your apps.
Bottom line, wait a little while then go ahead and buy. Next major upgrade wont be in until early 2010.
Ah, and enjoy it, by all accounts its a great computer.