If you dont get something large it wont give you much improvement over the Antec you already have...
My suggestion :http://www.newegg.com/Product/Product.aspx?Item=N82E16835608024http://www.newegg.com/Product/Product.aspx?Item=N82E16835181032http://www.amazon.com/Thermalright-Silver-Arrow-SB-E-Extreme/dp/B009ARE2YE/ref=sr_1_3?ie=UTF8&qid=1354368783&sr=8-3&keywords=Silver+arrow
Technically the H100i is the best but only at max refs on the fans, behind it is the Silver arrow SB-E Extreme(my fav as its more quiet than the H100i and performs better if the h100i is not set to drown out your thought.)
The D14 is good too and a little smaller.
My opinion, get the H100i if you don't want to crowd your mobo and case.
If you want something quieter and very good performance but large, get the silver arrow.
Something smaller and a not so big performance increase from your Antec, get the NF-D14...