What happens to my psp if i download a custom firmware?

Nothing, you just downgrade, nothing happens to anything you've saved (But you'll need to update to cfw) also, you might brick (but thats 0.000002% chance) during downgrading/updating

if you downgrade your psp correctly and install CFW, then your psp should be fully functional. the only difference is that the psp will recognise ISO format files on your memory stick, in effect, allows you to play downloaded games. the CFW will also allow you to play PS1 games in an EBOOT format. in addition, the CFW will also allow you to play homebrewed games. All your videos, music and photos should remain the same. you should still be able to play all UMD fomat disks too.

Nothing u must hardmod your battery or buy one that has been already and for the magic memory stick you need windows xp dont work on on 64 bit xp and be sure u back up all your stuff caz u have to format the memory stick and you can play downloaded games wen u get cfw it dont work on new slims only da fat and older slims

not much i done it b4