If you have a voice-enabled modem card, you can also set up a connection server with your high-speed internet. Or you can spend hundreds of dollars on a high-speed adapter on eBay. Whichever you prefer.
If neither are options, you could get a free dialup ISP like NetZero and connect through your home phone. That is, even if you have a Dreamcast modem (you should, they are supposed to come with them iirc)