Second on the Delta Triple Blade Low Speed from sidewinder. The design will give you less of a dead spot in the middle of the heatsink.
One thing though, I don't know if that heatsink holds the fan on with spring clips or screws...if it's screws, you'd have to replace them if using the DTB, the stock fan is 25mm thick whereas the DTB is 38mm thick. Should be able to find the same size screws at your local Ace, Lowe's or Home Depot though.
-z
Edit: when ordering from sidewinder, or anyplace for fans, look through the rest of the site for things like screws, sleeving, etc. so you can pad your order to save on shipping.