If I ever have to handle big text files I always switch over to TextWrangler. Same under the hood engine as BBEdit, but free. As in beer.
The one thing I would improve about TextMate would be to give it a command / insert mode like VIM (odd because the thing is inspired by emacs) and give it better large file handling ability.
BBEdit will work on really big files and intelligently swap in / out the portion you are working on.
EMACS also has paging.
Have you considered adding RAM?