In reply to spenser:
I'd suggest thinking long-term: a rucksack should last you a good number of years, so it's well worth getting something that will do all (or most) of what you anticipate you are likely to require from a rucksack over the next few years.
So, if you envisage carrying bivvy gear at some stage - but not necessarily this year - buy a sack that will enable you to do that. Despite what some more macho threads and posts on UKC might suggest, a sac under 45l is unlikely to be suitable.
Again, because a sack should last you for years, it is better to pay more now than to end up having to pay twice at some later date because you have bought a less good quality one now. In addition to quality of fabrics and design, I'd also be aiming for simplicity of design. Finally, I'd suggest heading to your local independent climbing shop and asking their advice.