I'd get the 1.6, I did.
The 1.8 runs even hotter, and the 1.6 is plenty snappy unless a Core Shutdown occurs. I've had that only twice, under very heavy use ( i was trying to make it stuff up, by leaving it playing many movies, cpu test, parallels unstalling xp, time machine backing up, etc)
Coolbook, the program that lets you 'undervolt' the CPU so it generates less heat but runs at the same speed, has made my machine completely impossible to core shudown, and it's now almost silent, all the time.
BUT, everyone has reported better results with this app on the 1.6 than the 1.8.
Anyway, get the 1.6.
Can you get to a showroom to play with one?
This is plenty snappy... the only thing it can't do that i did on my old macbook pro is to play a game well whilst recording the video and audio of the machine to a video file with screenflow. BUt then i wouldn't expect it to. (it's a bit jumpy)
try warblade!
http://www.warblade.as Excellent game for air