Your E6400 Overclocks fairly well, I've been operating mine at 3.2 GHz; However, I do not think your Mother board may limit you. I would not upgrade the MB just to OC your E6400, You would be better off with a New I5-750 which easily OC's to 3.2 -> 3.6 GHz.
Here again you are caught in a catch 22, while I agree a memory Upgrade would help - If you are still not happy and decide to upgrade MB/CPU, you will not be able to reuse the memory.
PS - good luck with the poker. My son plays a lot, He won $27K two years ago in Vegas (World Poker Tournament), Not so luckly last year.