Well, she's only important if you are a Pyro yourself as she sells some of the better Pyromancies, she also ascends the Pyromancy Flame from +15.
On the subject of Quelaag, that fight is usually the first challenge as your weapons will be getting a bit out-dated by that point, the trick to, as with most things in DS, is timing. Get her spider attack telegraphs down and be sure to either dodge or block her sword attack, if you picked up Dragon Crest Shield from the Zombie Drake that is amazing for it (requires Master Key or going up the elevator by Quelaag's Domain).
Oh whatever you do do not go near the edges of the arena, sometimes her lava glitches and ends up above you completely blocking your view.
If you are playing online there are usually summon signs by the bonfire in Blighttown, alternatively if you are in Human Form a Black Phantom (Mildred) will spawn after leaving the bonfire which when killed is summonable, again in Human Form, just outside the boss room. She's invaluable as a distraction for the boss whilst you wail on her, just make sure you avoid her lava and fire explosion.
Oh and if you think Blighttown is bad, if you haven't played Demon's Souls, the Valley of Defilement is 100x worse. It's one of the few areas where Dark Soul's fails, it's atmosphere, although great, is just not quite bleak enough. However it makes up for this with places like Anor Londo or Ash Lake, the latter being my favorite location in any game ever.