It's all going to depend on the size of your kit, GT. People who do a lot of backpacking have often invested in small, light tents, mattresses, stoves and sleeping bags, whereas those who don't may be using kit that's considerably larger.
I'm lucky because my kit is small and compact, and I use a 50L Osprey Atmos whether I'm going for a night or a fortnight. If I was still using a big sleeping bag and a bulky mattress, though, I'd probably need a 60L sack. If I had a big tent the pack might need to be larger still.
The best thing to do is to take the main elements of your camping kit - tent, sleeping bag, stove and mattress - to a good shop and try fitting them into some packs. Bear in mind that you're going to need additional space for clothing, food, water, miscellaneous necessities and other little things that you might simply like to take (book, music, camera etc).
Whatever you get, though, you should probably look for something that has pockets for you to store the things you might need to use during the day: waterproofs, warm layer, hat, gloves, food etc - so that you don't need to open the main body of the sack until you get to the camp. That's not essential, but I find it much more convenient, and if it's raining heavily I definitely don't want to get the inside of my pack liner wet.
It's also worth bearing in mind that we're in the midst of a move towards lighter kit generally, and that includes packs. It wasn't uncommon a couple of years ago to expect to carry 3kg in the pack alone for a backpacking trip. These days it's possible to go much lighter, and that makes the whole thing much more fun (for all but the Herculean or plain masochistic ;-) My Atmos, for instance, weighs about 1.3kg, whereas the Osprey Luna I bought about 15 months ago is about 3kg.
If I were you, and depending on your budget, I'd be looking at Osprey, Gregory Packs, ULA and Go-Lite. There are other great sacks as well, but those are the ones in the forefront of my mind these days when I think backpacking sacks :)
Good luck!
p.s. if you let us know what kit you've got, and what your budget is, peeps may be able to be more specific.