1. Not good and no stable. Not good in reasons of it's outdated and old, therefore unable to run latest games. Unstable by means of plugins and homebrews. (in instance you decided to run some on your psp). You are more likely to run latest games so i recommend updating to 6.60.
2. If you're going to run ISO (game back up image), then you should have a cfw installed.
3. 6.60 ME/PRO
4. to have it updated to latest:
go here FaustNero
follow gen bypass guide on the guide tab.
update via system update on the psp's settings menu
grab cfw (also can be grab from the blog site)
run it on your psp.