SWEATIN' CELEBRITIES: It's already the second week of 1998, which means . . . well, it means most people have already given up on their New Year's resolutions. If you haven't, and if your resolution was to get in shape, you may be thinking of buying a celebrity fitness video for inspiration. You already know about Jane Fonda and all the supermodels, but there are a bunch of, well, alternative fitness videos on the market from the likes of-get this-Fabio, Regis Philbin and pre-makeover Barbie. So what does it take to get a fit-vid deal? According to People's POP Profiles, which measures celebrities' popularity based on a variety of attributes, you don't have to be fun or hot or unique, just overexposed.