You can get an Apple iBook in that price range--not sure if you want a Mac though.

Dell Inspiron 600m is reasonably sized and depending on how you configure it you should be able to get it down to your number. So is the Gateway M320.

HP Pavilion dv1000 is widescreen and is pretty thin and light too. Might be a little more expensive and it's crap for games, but if you don't play games then ya don't need to worry about that.

Just don't set your expectations too high when it comes to performance because anything at that price is not going to be super speedy.