Regarding tithing, the Old Testament passages speak also of tithing crops, herds, goods, etc - ie what you have, which isn't necessarily money. I believe a person can tithe their time as well, when volunteering in church (or indeed outside of church). After all, they aren't being paid.

I also think the whole concept of prosperity gospel is a gross heresy. God says he'll supply our needs, but the way some prosperity preachers carry on, they think all Christians should be millionaires (and if they aren't, they don't have enough faith, or some such nonsensical explanation).