Hi, I went up from 20 to 40mg in December 2009. It is indeed a big jump. While it was very beneficial to me and totally transformed my mood within a week, 40mg was just too much for me. I'd sit at my mate's house totally exhausted, feeling like all my energy was draining into a pit inside me. The prospect of going for a walk round the block was daunting at best.
I went down to 30mg after a few weeks and immediately I had more energy and no longer felt mentally "bogged down", so I was able to start enjoying life again. The only thing is I found it difficult to wake up in the mornings. My mate also said I was sometimes a bit manic, though this was probably partly relief at feeling so well after so long and partly him being unable to tolerate almost anything!
You should definitely try taking 30mg and seeing how you go.