Research is arguably the most important EC in med school. Definitely give yourself some time to settle in and adjust to the onslaught of material before you pile too much stuff onto your plate, though.
The search function is your friend, especially in the Articles tab. There are several threads on this. Most have the consensus that Board scores are most important; but some of the more competitive specialties do value research--especially if you've published. EC's don't carry all that much weight, from what I've read.