Jamar is all ready a better defender than Foster. He's been given the task of guarding the opponant's top guard basically every game, and has been fantastic at it.
Concerning Foster, agreed he's undersized, but look at Langdon. As painful as it is to remember this draft pick, he was taken strictly b/c of his ability to can the deep ball. He had no quicks, which translated to disaster guarding perimeter players in the league. Shooting threes is a valued skill in the NBA. Foster can hit the three AND has defensive quicks. He certainly won't be a star, but I think he might find a spot on some one's roster as a roleplayer.