What is the BEST cooling fan for my xbox360 slim?

Hi, i bought my xbox360 slim in september of 2010 and fell in love with it. When i first got it, i kept it on its side, but it made a weird noise so i put it up verticaly. Ive been recently noticing it gets really hot. What i want to know is 1) sood i put it horozontially or vertically 2) what kind of cooling fan sood i get (if there is one for horozontially if thats the best choice) Really appreciate the input!