I've got 4GB in the Macbook (which of course only recognizes 3GB). Memory was cheap enough when I bought the machine I couldn't see any reason not to max it out.
I haven't really looked at how much memory EyeTV uses; I can if it's useful.
I have definitely noticed that the Macbook fan runs full speed more or less constantly when I've got EyeTV recording. It runs a little bit less on playback.
Celos, what are the other specs of your macbook?