Double click on the ipod icon on the desktop. A setting window pops up. In your iPod "settings menu" just tick the box "manual synch". That means you always have to/can tell the iPod what to synch and what not.
This isn't an answer to your question, just a suggestion, but considering how much trouble you'll be going through to manage the music that way, you might consider buying another external hard drive of the same size as your iPod's drive and storing your complete iTunes library on it rather than on the laptop drive.
You should be able to find a decent 120 Gb USB external for around $75, less if you're patient and look for online specials.
Yeah these guys are right, you are crazy not to get an external drive, unless you consider your labor worth less than 50 cents an hour. Your music is not backed up on the ipod and you can't get it back out without buying a program like ipod liberator. If you rip your music to a hard drive in lossless it will be safe and can be copied once you start to see how nice it is to have all your music available at the touch of a button.
I had the same problem -- a 120GB iPod and smaller internal HD. I agree that an external HD is a good answer. I got a small portable one which is convenient (and you store other other files in different folders). But I had some trouble figuring how to setup my iTunes library on an external HD until I found a guide on iLounge.com's 201 iPod tutorials. Here is the link: http://www.ilounge.com/index.php/articles/comments/moving-your-itunes-library-to-a-new-hard-drive/ It works like a charm.