In May of 1996 the E39 was delivered in the US as a 1997 model. There was no 1996 five series BMW. I have heard of a E60 in 2003 but it could be a 2004 model. If that pattern were followed we wouldn't see an M5 until November 2006 as a 2007 model.
Having said that, with competition being what it is with Mercedes pushing 55's out the door almost immediately in every model range, BMW may ramp that up some what.
If I were you I would buy the current M5 because this is a great car and with gas prices rising, talk about speed getting increasing attention by the media, congressional focus on cafe standards and just the general economy by the year 2006 BMW may not decide to build M5's at all. Why run the risk of missing the boat on the chance that the next M5 will be 500 to 550 hp and have to wait five or so years. Also, when they do bring the next M5 I can assure you the cost will be significanly higher.
Just a thought. Remember a bird in the hand is worth two in the bush.