So you have the original release of D2:ED? You will only need one of DKS or FoV (on Xbox there was no separate release of FoV, so the only option to play the add-on is DKS).
The Dragon Knight Saga contains both the remastered D2:ED and the add-on FoV. Saves from the original release of D2:ED are not compatible, though you can start a new level 35 character at the beginning of the add-on.
If you install Flames of Vengeance after the PC version of D2:ED, you can continue with your original saves, though some of the changes made in DKS (redesigned flying fortresses, changes to enchantments, enemy placement, loot, etc) will not be applied (some engine and graphics updates will). After you finish your current character, there is a patch to convert D2:ED + FoV to DKS, if you want to start a new character to check out the changes in the remastered version.
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