Your level is fine for Darkroot - the area after the Titanite Demon under the Parish, although I might save the boss Sif for after O&S, but you can certainly do the Moonlight Butterfly boss, it's one of the easiest.
Once you beat O&S you get the ability to warp between bonfires, but before that have you opened the cage lift in Sen's Fortress? It takes you from top to bottom, so it literally takes a minute or two to get from Firelink to Anor Londo.
The key on the roof of Firelink is for a door back at the Asylum for a very useful item - a word of warning about the Asylumbe careful when you walk through the room where you fought the original Asylum demon boss, the floor gives way and you face a stronger version of the same boss - best to avoid the center of the room until you have gone to one of the bonfires at the Asylum to save you going all the way back there in case you die. Well worth killing the boss though as it drops a slab!
Yeah I found the key on my first run through Sen's. I went through Darkroot Garden and killed that butterfly, pretty nifty. It was eerily beautiful, I kind of feel bad for killing it. I'm thinking about going to the asylum, but I also want to get O+S over with now that I have enough humanity to summon someone (and to survive an invader or two).
Also, is it worth killing the undead merchant for his katana? I really want that weapon, and it seems like the only guy who sells it takes a lot of souls to get to.