I think if Colorado is won by Gardner, then its a wave. If the GOP can win an uphill battle against a decent incumbent, that's usually a good sign (Like McCaskill winning against Jim Talent in '06). I agree that its somewhere around R+9 or 8.
Iowa would probably fall for the GOP before Colorado. There are 7 vulnerable democratic senate incumbents in states that voted for Romney in 2012, so assuming that all of those seats flip to the GOP then only one of NH, VA, MI, IA, CO or OR has to fall in order to make it a "wave." Iowa seems the most likely to do that.
