The domain should not matter. It sounds like you did not succeed in restoring the database correctly. You must make a backup on the sql 2000 machine and then restore that backup on the sql 2008 machine, it cannot be done by importing objects, it must be done by restoring the db. Your db user should be dbo on the database or he may not see the objects which could cause the stored procedure not found error.
It should work fine by restoring the db, but if it is a small site with only a few pages and you are having difficulty restoring the db then you could just re-create the content with copy paste, but it is better if you can just restore the db if you have lots of content and existing users.
I would not try upgrading at the same time, I would migrate to the sql 2008 using the existing version you have then after getting the site up and running on the newer db platform proceed with upgrading to the latest version of mojoPortal. Doing both at the same time is more likely to be difficult.
Hope it helps,
Joe