In reply to Gav M: I've used a Karrimore (now OMM) 25litre sack for years - and find it perfect for the job. I would replace it with the same every time, very comfy to run with.
25 litre size carries my Terra Nova laser comp, sleep mat, down bag, cooking kit, food for 2 days, some spare clothes, waterproofs, and all the sundries I need, usually comes in around the 12lb mark without me being silly (minimalist for the sake of it). The 32 litre would give you plenty of space for extras and certainly no need to go bigger than that. Moley.