Disgusting.
Quelaag's sword isn't great for PVP because of the split damage. But if you do use it, be sure to have 10 humanity in your counter for max damage. If you bring INT to 25 you can enchant your weapon with crystal magic weapon if that's your thing. What's all the attunement for? DEX/INT builds are very popular, you will run into a lot of players with optimized stats using 40INT CMW on katanas with 40-45DEX. 45DEX will increase casting speed. I'm not a fan of the sorceries in PVP but I really like pyromancy, and it gets the casting bonus speed as well. I love great chaos fireball (for unlocked throws), chaos/fire whip, and fire tempest.
Basically, around SL120 you can do better than having a dedicated DEX character because you have enough levels to bring your INT up for a dex/int build or STR up for a quality build. Your stats look closer to leaning toward a dex/int build. You might have some wasted points in attunement/int/strength, but it's not a big deal. You can level up a bit past 120 and just wait to get invaded in the popular spots and see what works best for you. There's still people at level 200-700 waiting to get invaded as well, but I'd imagine you would get more action if you hover around 120.
Might want to put your stats into this character creator so you can see how different weapons/armor will work out with your setup:
http://mugenmonkey.com/darksouls/
I suggest more VIT. ~1200HP is pretty low for SL120 PVP.
As for weapons, check out the silver knight spear. It has excellent range and good damage. At fast roll or ninja flip it can be very difficult to deal with. Also consider balder side sword, great scythe, winged spear and the ever popular uchigatana. Take DEX to 40 for the best scaling, and if you can spare the levels and use spells or pyromancy, take it to 45 for the best casting speed. So if you kept your current stats I'd put 5 levels into VIT and 5 levels into DEX, and you'll be at 125. Consider using Ring of Favor instead/in addition to those 5 levels of VIT as well.