Yes, you can use both Sega's ISP and your PC's ISP. Seganet is faster though, compared to FreeIsp's, I have noticed.
I would reccomend using Seganet. They have a trial version. Go Rent or Buy NFL2k1, Quake 3 Arena, and then you can get a 50 hour free trial. Also, If you buy the Official Dreamcast Magazine, you can get the browser disk, which has a free trial of the browser