This map gives a 273-265 victory for Dean by enabling him to not lose NM and IA and by carrying CO.
Dean was an antiwar candidate and would have ran on an antiwar platform. One thing Democrats did not take into account the degree to which the country was quickly turning against the Iraq War even in 2004.
A ticket of Howard Dean and an anti-war running mate may well have kept IA and NM and flipped CO. NM was trending Democratic anyway. IA and CO have long been states where there has been significant dovish impulses, and CO has been trending toward cultural liberalism for a while prior to 2004.
I don't know if Dean would have won OH. OH was more Democratic back then, but I can't see Dean cracking OH in a year with a GOP President up for reelection.