1. You don't need to flash it. Just install CSSU stable (or testing if you want to risk a reflash). Familiarize yourself with the cssu wiki page (including the strange hildon desktop problem)
2. What you can't do: aptitude gui isn't working (but command line yes), there are some apps that are not on n900. But you'll be happy, as there's Easy Debian, which will make you able run all the open source progs that are on Debian! (With LXDE too!)