I don't have any real world data for multi-boxing/Dalaran, so get that somewhere else.

As far as performance and compatibility the latest Phenom IIs may be exactly what you're looking for.

Check out these two articles (and several others at the anandtech link):
http://www.anandtech.com/guides/showdoc.aspx?i=3552
http://www.anandtech.com/cpuchipsets...oc.aspx?i=3551

From the first article:
In January AMD launched their new quad-core Phenom II processors that were compatible with existing AM2+ motherboards and DDR2 memory. The new Phenom II processors were the first truly competitive CPUs since Intel's introduction of Core 2. The Phenom II 940 and 920, then priced at $275 and $235, performed better than the equivalently priced Intel Q9400 and Q8200. However, the first Phenom II processors support only DDR2 memory and can only be mounted on an AM2+ motherboard. A few weeks later AMD filled out the Phenom II line downward with five new models with integrated DDR3 and DDR2 support. These new AM3 processors could mount in socket AM3 and support DDR3 memory or mount in AM2+ and support DDR2 memory.

This looks like right up your alley. Read the second article and check up on compatibility, see if your mobo is compatible or can be with a BIOS update.

Designing the new chip to be able to physically be compatible with AM2+ or AM3 sockets and being able to run DDR2 or DDR3 is brilliant IMHO, from both an engineering and a marketing standpoint.

edit
This link is also excellent info on the Phenom IIs
http://www.anandtech.com/cpuchipsets...oc.aspx?i=3492