First-Time User? Ubuntu. If you're somewhat familiar with Linux, but want something powerful and flexible, then Fedora Core. I'd recommend using version 5 rather than the latest, FC6, because in my experience and that of others, 5 has less bugs, therefore less headache to use and set up.
Ubuntu's package manager is a piece of cake to use, so if you're not used to installing things by hand, or don't want the dependency hell that comes with RPM's, I'd recommend it on those grounds too. Ubuntu is just simple and easy to use, but like Fedora Core, I'd recommend using version 6.06(Dapper Drake) rather than 6.10(Edgy Eft). The latter is not as stable of a release, meaning it has more bugs. The tradeoff is that it does have slightly better hardware support due to cutting edge drivers and software.
It all depends on whether you want something that's easy to use and learn on, or if you want to dive right into bleeding edge software. You'll see a lot of distro's out there, but I tend to recommend the "big" ones because they're usually the easiest to get going right out of the box, and there's a plethora of support for them.
In fact, your hard drive is rather large, so you could easily afford to install several different distro's at once, then decide which one you like best.