DO NOT spend $300 on a CPU for that motherboard. LOL You're using the LGA 775 socket, which is now out of date (not obsolute, but old). New CPUs aren't being made for that socket anymore. It's now been replaced by 1156 & 1366 sockets (i5 & i7 CPUs).
A Core 2 Quad CPU would be a good upgrade if you use modern versions of Photoshop and other such programs, as they can use quad cores. Otherwise, something like an E8400 or E8500 would be very good dual-core chips.
You should be able to get into any of those for well under $200. Considering you can buy an i5-750 (GREAT CPU) for $199 (although you'd need new motherboard) it'd be a waste to spend more than $200 at this point.
You could also consider a good cooler for your E6600, and overclocking it. The E6600 should overclock to 3.2Ghz or more fairly easily as I recall.
Asus says it is Intel Quad Core Ready (which is socket 775 chips, or Core 2 Quad). Will NOT work with i5 or i7.