In light of some of my other posts and conclusions in previous topics, I have found this explanation of what jumbo frames are, selecting the proper size and the benefits of doing this to be pretty good.
It's a good read, check it out if your trying to tweak your network and squeeze out all the extra throughput possible... (particularly page 3)
enjoy
/g33k
http://www.smallnetbuilder.com/lanwan/lanwan-features/30201-need-to-know-jumbo-frames-in-small-networks