I'm not sure what the etiquette is for attending a reception if you skip the wedding. My guess is that it's uncool. I tend to think of the reception as the reward for suffering through the wedding. Therefore, skipping the hard part and only showing up for the open bar and high-brow food wouldn't be cool. The opposite is perfectly acceptable. You can attend the wedding and skip the reception, but that would be foolish.
As for dress, Eric nailed it. At the reception, things will be slightly more casual, but not much. You can probably get away with ditching the tie or the jacket, but not both.