Welcome to DC-Talk! Here's some recommendations for you...
Sonic Adventure - One of the cheaper and funner games on Dreamcast. The graphics haven't aged very well, but get this if you want a fast and good 3D platformer.
Crazy Taxi - Get this one if you're a fan of driving around like a maniac in GTA. This one's pretty cheap, too.
SoulCalibur - This game is all about the graphics. In fact, the graphics here even surpass the arcade version! Many drooled at the intro when this first came out (especially when hooked up to S-Video or VGA). If your friends say PS2 is better than DC, show them
this.
Jet Grind Radio (or Jet Set Radio for other territories) - Fairly inexpensive, colorful, and most importantly,
fun. In Jet Grind Radio, you skate around cities and spraypaint on objects to claim your territory. Just watch out for the coppaz!
Marvel vs. Capcom series - Great fighting series! Most people even go on to say that they're the best on the Dreamcast.
Power Stone series - People were begging Capcom to make a home console port of the arcade hit Power Stone. And when we got it, we loved it. And with Power Stone 2, they updated it so four people could play at once. Sweet!
Seaman - A rather interesting game...you talk to a fish through a microphone in the controller, and he responds. Once in a while, he will even ask you personal questions. One of the more expensive ones, but worth the money.
Phantasy Star Online - You said RPG? This one from 2000 will boggle your mind, questioning "How did they have this technology in 2000?" Well, it's all thanks to the Dreamcast's built-in 56K modem. Today, you can go on private PSO servers (ex. Sylverant) and still find people playing!
Space Channel 5 - A fun rythm game for the DC and a launch title for the system. If you liked PaRappa on PlayStation, you'll fall in love with this.
Ferrari F355 Challenge - One of the most realistic driving games of all time. Get this if you're a fan of driving simulators.
Hint: Type "DaysofThunder" into the hidden Password menu and you'll unlock five bonus tracks, including one from Gran Turismo 2.
And finally, one of the best, if not the best DC game out there, it's...
E.T. FOR THE--wait, what's that doing there?! Okay, okay. Take 2!
And finally, one of the best, if not the best DC game out there, it's...
SHENMUE! - I'd probably get bonked in the head if I didn't mention this one. Great story, great graphics, great gameplay, great voice acting, great everything! This is a must-have and a DC masterpiece.
P.S. I think the DC version of SA1 is superior to the GameCube one in several ways...
..one of which is Chao Adventure.
I'm not actually in the Dreamcast scene just yet, I'm looking to get into it now though (over the next couple of weeks anyway) so I figured I'd join a DC forum. 
