I've used many distros over the past 11 years. My first was slackware, and it's always been solid -- my current favorite is debian. I'm also fond of freebsd.
I agree with Salem's advice to get another drive ... yes, 80G is a nice size. In fact, that's the setup I have on my main box: 2 80G drives. Then what you need to do is experiment with any distro that sounds interesting to you. Since the choice of a distro is such a subjective thing, only you can decide which is best, and you can only do that through experience. You can easily fit four distros on a 80G drive, and that's a good way to get that experience :-)
Good luck and have fun.