Well, in pure performance terms I expect the new thermalright si-120s to top dog or very close, plus a 120mm is gonna be a lot quieter then stock. They claim to have solved the compatibility problems the old XP-120 had too, which is always good news.
I thought about the Zalmans, but the sheer mass of the things made them look like they'd be a right royal ***** to install, let alone uninstall. Plus, the toss more mass at it approach just seemed inelegant. From what I hear, they're the quietest, but don't have quite the performance of the thermalrights.
As for noise, I didn't realize just how annoying the stock AMD coolers sound when they ramp up until I replaced it with an XP-90 and a panaflo.
And yeah, A64's overclock nice even on stock. For a while there it wasn't uncommon for 3200+ venices to hit 2.7Ghz on stock, but it's apparently gone downhill a bit. Still, having the option of running it nice and cool while still being quiet even while gaming or tossing some more voltage at it to see if you can get it even higher isn't a bad thing to have, nah?
Of course, that's kinda a moot point for the original poster. If cooling an A64 is like blowing on your soup to cool it off, cooling a P4 is trying the same thing with lava.