If you already have Ego Draconis (ED - the original game) then you need to get Flames of Vengance (FoV) if you want to continue playing with your original character - the Dragon Knight Sage (DKS) is both games rolled into one, but it isn't compatible with the original ED save games.
So if you've played ED, then buy FoV in order to continue your game. If you haven't, then buy DKS in order to experience the "remastered" (e.g. much better) version of ED along with the expansion.
The FoV expansion is apparently the same regardless of whether you play ED/FoV or DKS - the only difference is in the original ED part. There are even reports that FoV saves are compatible with DKS saves (once you've reached the FoV part of the game) although I can't confirm this.
