Lucy,
for what it's worth, these are my thoughts on rucsacs. As was said before buy the best you can but the most expensive is not necessarily the best.
You can't go far wrong with a berghaus as a first try, they are basic but very tough and can be picked up very cheaply on ebay if you can wait a while.
One option is a cyclops roc approx 60 litres, this is based on a climbing sac and has no side pockets. this means it is a slim rucsac and will not easily get caught on branches and bushes. you can extend the capacity by adding side pockets or strap roll mats, bivis etc to the sides.
The other option is the crusader 65-80 litres, pretty much the roc with zip off pockets that convert into a daysac. I have been using these for over 20 years and they work for me. I have had plenty of other rucsacs, issued to me and bought by myself and keep going back to the crusader. sold my DPM one on ebay and bought a green version in great nick for £30, 2 weeks later.
The final option is the vulcan, 100 litres and a monstor. Great sac but then you are tempted to pack all you own and regret it very quickly. plenty of these come up on ebay.
The most important thing to be aware of is the fact that they come in different frame sizes 1-4, depending on the model.
Size 2 - 5'3" to 5'8".
Size 3 - 5' 9" to 5'11".
Size 4 - 6' to 6'6".
sice 1 is found in the sacs designed specifically for women.
They fit close to the back and the frame can be shaped to your back. The plus of this is the stability over rocky ground, the minus is a sweaty back, only a problem if you are stopping and starting and don't like the cold feeling.
When buying, look for a small white tag sewn on the back, between the shoulder straps with a little number on, this refers to the frame size.
I could probably knock up some photos of my crusader if you want.
Hope this helps and prompts some more posts on rucsacs.
Rob
Thanks very much for that great post! I think I'm looking more along the lines of 40-50 litres, if you have any suggestions for them?