Simply putting in a blank SD card will have no effect. You have to free up as much space on the internal flash as possible - the SD card is simply a catalyst for that action, because it allows you to do so without deleting the files. With an SD card inserted, you're given the option to install programs (from CAB installers) on the SD card rather than on internal flash, and you should always do that when possible (there are some poorly written programs that refuse to work in that way).
In any case, even if you wiped the internal flash clean, you still wouldn't have enough RAM to run BNUBot, so you shouldn't bother trying. The lower limit for running it functionally is something like 42MB, but that would cause nearly constant collections, so it would absolutely crawl. I wouldn't recommend attempting to run it with less than 64MB of available application memory, with peak performance at around 100MB.