As others have said, you can set up the iPod to automatically sync, or you can have it manually sync, where you pick and choose what songs you want on or off of it or your computer. With 15G of music, manually syncing will get tedious though!

I would suggest getting a cheapo external Firewire hard drive, 15 or 20G, dedicated to your music library. This way you don't have to worry about your music taking up space on your primary hard drive. It looks like you can pick one up for about $100. If you have a desktop with a spare drive bay, you could get an internal drive for about $50, but you'd need to install it (not really difficult).

It sounds like you have a Wintel system. iTunes for Windows will be out the latter part of this year (which is approaching). Having all your music on the separate drive will work particularly well for iTunes. Also, iTunes should really improve the integration of your iPod with your computer, hopefully to the level that Mac users enjoy now. --And you'll get access to the iTunes Music Store, which also integrates nicely. There are other online music stores out there, but so far, I like iTMS, because it's so seamless, easy to navigate, and the DRM is the most friendly.