Ask yourself this... how many other people do you know who could run a compatible AV chat program? If you know a lot of people, then consider the iSight. But as others have said, you might use the iSight occationally, but the RAM will help every time you use the computer.
But, with that said, I must echo Mr. Mart's comments. The iSight is really cool. I have one for my desktop, and I'm seriously considering getting one for my on-the-road iBook, too.