Get the phenom II. It's cheap, overclocks great, and if you get a good board you will be able to drop in a faster AM3 CPU latter on. On the other hand there won't be any new faster CPUs really for LGA 775
As for Sotsu, I'd say that if you can get some decent overclocks on your QX6700 then you probably won't notice too big a jump getting a PII 940 and you need to spend more money on a new board for it. It would be an improvement if you overclock it, but I'm not sure it would justify the price. For you I'd say that if you're board can support the newer quad cores just get a Q9550 and overclock it if you really want a speed boost.