You want to upgrade to a new bass or upgrade the parts on your existing bass? If you were my student I would suggest a new bass before modding. The RBX are already fairly solid for their price point. You will end up putting $250 in parts and pieces to still have a $100 guitar. Not a good investment in my books.
My first bass was an RBX170. As Diabolous mentioned, they're already pretty solid. What don't you like about it?
'Upgrading' w/o a specific purpose may not really be upgrading. As mentioned prev'ly twice- "they're already pretty solid. What don't you like about it?"?