I have a dreamcast, however, I did not put it in my cab, because I did not want to be constantly changing discs.
There are alot of great 2-d fighters for the dc.
My favorites:
Marvel Vs Capcom 2
Capcom vs SNK millienium fight
There are also good 3-d fighters
Soul Caliber is a must have....it will look great on your cab.
Dead or Alive 2 - I love this game. Very simple and fast paced.
virtua fighter 3 - I don't like it nearly as much as Soul Caliber. But you may consider it.
If you have lightguns for your dc, which I would highly recommend, you should get:
house of the dead 2
Sega Smash pack -- Virtua Cop 2
Mission something or other...The other dc gun game. I will get the real name when I get home from work. I forgot.
If you have a 4 player cab, I would reccommend powerstone 2. It is kind of like 3-D smash bros, and it can be a lot of fun with 4 people.
Gaunlet Legends is another good 4 player game.
VirtuaTennis 1 and 2 are also very good 4 player or even 2 player game.
Bangi-O is a quirky scrolling space shooter. It may give you a siezure, but it is fun.
That's all I can think of for now. A good place to learn about dc games is here:
http://www.gamespot.com/dreamcast/reviews.html