If you are a hardcore fighter fan, get a Dreamcast.  Seriously, it has so many fantastic fight games its not even funny.  2-d and 3-d.  Plus DC stuff is cheap, it has better graphics that a Ps2 (ps2's superiority was a lie), and there are some fantastic fight game controllers for the system.
As for GC, if you have SuperSmashBros and Bloody Roar you're off to a good start.  However make sure that you get the GameCube version of Soul Caliber 2.  I have now played all three and I can say this, Link is the best new character in the game!  Also the GC analog stick facilitates 3-d movement around your opponent much easier than a D-pad.  
Capcom vs Snk2:EO is also a great 2-d for GameCube.  However you might want to look into one of those cheap psx/saturn controller converters for GC, as some people cant play 2-d without a D-Pad.  
As for X-box, its only good 2-d fight games are titles that were out on DC two years earlier.  Plus they look exactly the same.  The only difference is that Capcom vs. Snk 2 has online play.  However I'm not sure if online cheating has hurt the game, like it has done elsewhere.  There are some Capcom Arcade Emus that run really well on X-box, but you'd have to mod your system, and thats something I wouldn't reccomend.