Clovis was the first pagan king to convert to Christianity in the medieval period. What did kings like Clovis stand to gain from this arraignment? Was it just to gain favor among his Christian subjects? Or was there more to it?
Also, later in medieval period it seems that there are two ruling classes. The nobility and the clergy. Nobility ruled in secular matters of state and clergy ruled in religious matters. What did each side have to gain from the others existing? What was to stop a king from appointing himself as religious authority (like Henry viii). And, what was to stop the church from raising an army and ruling.
What did Constantine have to gain by converting to Christianity? I know the story that he had the cross painted on his soldiers shields and he became Christian to thank god for helping him win a battle. But, this story sounds apocryphal. He may of had the symbol painted on his soldiers shields, but it I think it was probably because his soldiers were christian. And, he made Christianity legal to thank the soldiers who fought for him. Not because he had divine help.
I think the comments so far have underestimated the importance of faith in late antiquity. We shouldn't just see everything in terms of what is 'useful', as there is no reason for us to see Constantine or Clovis' conversions as entirely cynical decisions. Constantine for instance had a Christian mother and Christian advisors in his court already. Combined with his pre-existing solar monotheistic beliefs, it was a very short leap for him to become a full-blown Christian. At a research seminar last year Peter Heather, a historian of late antiquity, also noted that we have no evidence that Constantine wasn't a Christian before the Battle of Milvian Bridge - his public proclamations of his solar monotheistic faith (on coins and panegyrics mostly) after all did not necessarily represent what he believed in private! For additional information, check out my comment on Constantine's conversion here.
As for Clovis, I think the same applies to him as well. He was already intimately acquainted with Christians (such as his friend Bishop Remigius) and ruled a largely Christian polity, so it does kind of make sense for him to convert. His sister had already converted to Arianism and one of his responsibility as a king was to protect the Roman people under his rule, so Clovis was definitely very familiar with the basic tenets of Christianity. Moreover, his most immediate neighbours, the Visigoths, the Burgundians and the Ostrogoths, were Christian as well. We will probably never figure out exactly how cynical or devout he was, but there were certainly plenty of reasons for him to convert either way!
this isn't here nor that but as the story goes it's not actually a cross, it's a Chi_Rho according to Eusibius.
http://en.wikipedia.org/wiki/Chi_Rho
also i would point out that there weren't two ruling classes. The nobility and bishops usually came out of the same families or at least sorts of families and tensions between church and state were widespread throughout the middle ages (though that's going way off topic).