If John Kerry (D-Massachusetts) would have won the presidency in 2004, his Electoral College victory would have been the upper-300s in electoral votes (winning in the upper-single digits, and taking John Edwards's home state of North Carolina; probably a 10-point shift).
Having mentioned that, Election 2004 would have ushered in Congressional Democratic victories to win back the House in that election plus increase the Senate with Majority Leader Tom Daschle (D-South Dakota) not having been unseated.
2006 midterms would've depended on the political climate. But one thing's certain: it is rare for a president to see his party gain seats in either the House or Senate in the second year of his first term. Again, it "would've depended on the political climate."
You're assuming a popular vote win nationwide, and thus a swing of at least 1.5 to 2 million in the overall popular vote. I recall Ohio being won by something like 120,000 votes for Bush, so a theoretical shift of just 60,000-65,000 would have won the whole thing for Kerry. Thus, the rest of the election could have easily turned out exactly the same (save for a couple of House or state seats in Ohio that "could" have turned had Kerry pulled in just a few more votes his way.
