I think Quelaag's sword is only good if you have a bunch of dex, and carry extra humanity, which adds to the scaled attack. I thought it looked cool as well, but it was quickly outclassed by my other weapons that were based on stats like Faith or Int. I
think my two favorite weapons are the halberd and Claymore (or Longsword, if weight is an issue). Love the claymore's moveset, with the wide, arcing swing, or a heavy thrust.
You have to have the large ember (very large is for +10 to +15) and
you have to give it to Andrie (the blacksmith in the undead parish near darkroot and sen's fortress). You also have to have large titanite shard's to go to +10.
$100 says one of these things is missing.
Once you give it to Andrei then go to modify equipment.