Now that I know how to get all of the games, I'll check them out soon, see what the "plus" means. In the case of SVC Chaos Plus, it's just the same game, but will all the characters unlocked.
*EDIT*
To add to that, Double Dragon Plus is the same game but with the boss characters unlocked (note: THIS double dragon is a fighting game rather than a beat 'em up... and it's loosely based on the movie it seems... and it's actually quite fun).
King of Fighters 2002 MAGIC II seems to be the same game, but with unlimited super bar and I THINK easier commands to execute supers, maybe easier combos or something... hard for me to say since I'm not terribly familiar with KOF in general.
Metal Slug 6 Plus is Metal Slug 3 but with more power-ups (for example, you start with a thunder cloud).