2
1) Newbb 2, IPB mod, PHPBB port (the two latter ones need XOOPS core to be hacked to share user tables)
2) Newbb
3) I believe the IPB mod will not be updated in the future due to the payment charged by Invision.
4) I have not seen Simple Machines ported and vBulletin is not legal to port as it is not GNU GPL licensed.
Play around with the options in the forums here on xoops.org - that's Newbb 2