yea 300BB is good for limit full ring. I think its 500BB for 6max.

Im not a limit player but Ive heard that 2/4 is where it gets tough.

Also I think most of the Limit players will tell you to switch to 6max. Good players will have more of an edge there but varience is ALOT worse.
If you decide to take the 6max route then stick at 0.5/0.1 but go to 6 max when you are rolled for it.

You are best off talking to the kind folks in the limit forum about that kind of thing though.