Definitely go light weight, lots of UL packs only cost between 100 and 150...check out:
-http://www.ula-equipment.com/
Someone already mentioned ULA above, which may be the most popular brand of packs on the PCT, they are known to be ultra-light and durable
-http://www.gossamergear.com/
Gossamer gear has a cool pack comparison that you can click on, also saw a lot of these on the PCT, but some PCT hikers said they are not so durable, but when you're consistently carrying 15-20 pounds of food the pack can gets beat up way more
-http://www.sixmoondesigns.com/default.asp
Six Moons also has a pack comparison link.
-http://www.mountainlaureldesigns.com/
Mountain Laurel Designs has some very intriguing products. I have been eyeing the prophet and revelation packs
Whenever buying from the ultralight companies, be sure to call them up and ask as many questions and "dumb" questions as you can, all the ultralight companies have great customer service. Things to consider are how much weight you plan on carrying? Do you carry your tent inside your pack? Do you carry your pad inside your pack? Hope this helps!