So far as disk space is concerned, the first thing you should do is boot off those OS X DVDs, format the hard drive, and reinstall Mac OS X. Be sure to select "Expert Install" or "Customize" during the installation, and deselect all the junk you don't need. Get rid of stuff like drivers for printers you don't own, localization files for languages you don't speak, extra software such as iMovie that you won't be using. That alone will probably give you an extra 4-6GB, depending on what you remove.
If you'll be leaving the MacBook on all the time, or using it for an extended duration, it'd be a good idea to put it on a hard, flat surface when using it. (This means not setting it on the carpet, or your lap, or a couch cushion, etc.) The Core Duo processor can generate a lot of heat, and it'll build up quick if the system doesn't have a little space underneath it. I'd recommend putting it to sleep when you're not using it, just to save power, and to let it cool when you won't be using it anyway. Leaving it on 24/7 shouldn't be an issue, however, provided it's properly ventilated.
Also, it's Mac OS X, so restarts generally aren't needed unless you're doing a Software Update, or something manages to completely hose the system (which is fairly rare, in my experience.) Mac OS X is a Unix-based OS, so it knows what it's doing when it comes to memory management and such.
Hope you enjoy your new MacBook as much as I'm enjoying mine