Well I'm heavy dex now, like 35 in dex. I don't know what's better atm. Using my raw +5 butterfly spear, or leveling up this demon spear, It's hard to tell how good the demon spear is since everything in anor londo resists lightning (I think.)
Any tips on what weapons I should be looking for as a Dex build? I've not really begun going down a magic tree yet, but that's my next step once dex is at 40.
In addition to Booshka's recommendations, all of which are quite solid, you could consider Quelaag's Furysword. It's only got B scaling with Dex, but it also gains damage for every point of stored Humanity up to 10. It's basically a Chaos weapon that can also scale with stats, in other words.
On the magic front I'd say to go with either Pyromancy or Sorcery. Pyromancy is great for everybody because it's totally stat-independent, only the upgrade level of your flame matters. If you go that route you leave plenty of stat points free for Endurance, Vitality, or anything else you care to pump. Sorcery gets you tons of offensive power as well as Dusk's utility spells like Chameleon and Cast Light. More importantly, Sorceries aren't dependent on Covenants to the same degree that Miracles are. A Faith-based character, I think, is better suited to a later playthrough when you know where to go to join the Miracle-granting Covenants and don't need to worry as much about maintaining your membership and leveling them up to receive all the Miracles.
Being expendable makes me feel a bit better, but not much. Now that I think of it, I'm not sure I managed to cut off the Gargoyle's tail. I tried to do it. I think I did it. But I'm not positive.
As a phantom, you could partly ruin someone's playthrough by denying them weapons and armor.
It's true, you could. You'll occasionally run into somebody who just blitzes the boss with no regard for getting tail drops, or someone who doesn't wait to see who the host wants to kill first against O&S. Those people are kind of dicks, and the best solution is to just not be one of them. The overwhelming majority of people I encounter in co-op are quite conscientious about making sure to cut the boss' tail if it has one and to let the host choose who to attack first against O&S. The thing about co-op, though, is that after a fairly short time doing these things will become second nature, and cutting tails will become practically a reflex. There is not a single boss in the game whose tail I can't have lopped off within 60 seconds of the fight starting, and it's all thanks to a steady diet of jolly co-operation.