If you don't believe that the New Testament is the operative authority, you're not a Christian. I guess you'd have to be considered Jewish. Not that there's anything wrong with that. :)
I am not stating that the full requirments of the Old Testament laws about interest should be in operation. But I do think there is matter to ponder - this is something pointed out, both in the law and the prophets, as against good morality. The question, of course, is what is usury, what is interest (is it any return on money, or an immoderate return, or a contract which allocates risk only to one party?) etc.