I think 3.50 is the latest jailbreakable firmware. Not 100% sure though. 3.55 is the latest update that's required for PSN and I'm probably just going download that, since Demon's Souls is 50x better online.
Yep,you have to downgrade to 3.41
You could consider PS3,magus
Hacking is easy(although latest games don't work yet)
-just plug in some cheap usb stick,download games to hdd
-no warranty loss,no special PC chipset needed
-easy revert to previous state
Some not too old,reasonably priced PS3 would be good.There is a catch(for now),if it has firmware higher than 3.50...not hackable