well, im not the most experienced, but i know the first thing people are going to ask you is:
how long are you traveling...most obviously prefer a bigger pack when traveling for say, 2 months, compared to 2 weeks.
i myself have a 3 weeks trip planned followed by a 1 month trip. i bought a 5000ci bag. it will be tight, but from what i gather, packing light is the best way to go. you can always pick up some stuff if you need it along the way.
hope that helps.
also...the poeple on these boards have a tremendous amount of knowelge on where to get great deals. i'm not the brightest on that either, but i got my bag online at budgetbags.com. i have seen nice deals on REI at their outlet page. good luck with your search.