When the volkswagen golf mk 6 comes out will that decrease the value of the mk 5 in terms of the resale value.

Yes, it will if the MKVI even makes it to the U.S. According to a few news reports VW has said they are still undecided on whether or not they will bringing the MKIV to the states. Apparently, since the value of the dollar is going down so drastically and the cost of importing have gone up so much, they are reconsidering the US as a good market for the Golf. Hopefully, it will make it. But, back to your question, there are very few cases in which the new older model holds resale better than the new models.