Share 'Marathon before an Ironman'

I am doing IMCDA on June 21, 2009. I've already planned on a half-IM on April 18th as a tune up race and just to see where I'm at. Would you suggest doing a marathon or would the recovery take away from any possible benefits? I don't need the mental edge because I've done more than my share of marathons, but I do want to do everything I can to make my IM the best ever. Thanks!