With our current personnel I think a 3-4 would be a bad idea.
In a 4-3 the down lineman eat up blockers and leave the linebackers more or less free to make plays. This is why we can have smallish linebackers and get away with it - as long as they're quick. With a 3-4, the interior linebackers have to handle the linemen more often. I think Dillard may be big enough to handle something like that, but not McKeon. I also remember Ruud not liking to take any blockers head on, which he may be asked to do in a 3-4 defense.
I think we probably have the nose tackle position covered. With Suh, Barfield and "Beast" Johnson, that's three big bodies to rotate - although I don't know if the latter two can really play. We need Potter to step up, 3-4 or 4-3, to man the base end position. Pierre Allen may be able to play there as well. But my personal feeling is that we'd be wasting someone's talent on the other side of the line. Steinkuler could play there, or maybe Pierre Allen, but Turner seems like more of a rush end player to me, not someone who will hold the line at the point of attack. But maybe they'd put him at outside linebacker, I don't know.
I think a 3-4 could work effectively, but we'd probably need some different types of players again.