look for a hsf w/ heat pipes, they are by far the best way to cool (using air).
you may want to try reapplying your AS-5 before you decide to get a new hsf. it may be old/crusty, new (not set up completely, it takes like 100hrs+ several heat cycles to set up completely), or just not applied evenly/completely
Cheapest Heat Pipe I could find
Extremely good 3rd party reviews out for this one
^^^Lotsa heat pipes
My personal Fav (i have the VGA cooler that matchs but not the cpu cooler, very happy w/ it)
Scythe SCNJ-1100P and the Ninja put up slightly better heat transfer numbers than the Zalman, but i think the zalman looks way better under the hood
remember, these things are like 3x bigger than they look in the picture! make sure you have room for them!
u can trust my opinion, im a heat transfer engineer