I'm using a 2600-K and have it slightly overclocked at 4.4 Ghz--just because. I don't know if it's helping much if at all in AH. Reason I say this is I run Aida64 in the background and have speeds and temps, etc., displayed on my G-15 keyboard that has a little LCD screen on it. Like a little dashboard. When I'm playing AH, in all circumstances, it doesn't seem to get much past 20% load. Now my GPU (MSI R6950 2GB PE/OC) yeah, it'll get close to max'd out when things get busy with explosions and smoke and puffy but never the CPU. Not even close.
I'd say unless you're looking to use it for something other than AH that might require the extra HP save the money. Put the difference into a kickass video card that'll be more help. I'm thinking my 2600K will have me "future proofed" insofar as AH goes for at least a little while.
I'd add that some mobo's have had problems after updating the BIOS in order to accomodate the newer Ivy Bridge CPUs. My Asus P8Z68 Deluxe is one of them. I CAN run one on my board but it's apparently more trouble than it's worth. At least for now.