The KFN5-D supports Barcelonas, so yeah, you can move to eight cores for pretty cheap. While the 2360SE's would be at the absolute end of the upgrade path, these are just one step down from top of the line for just a few bucks:
AMD OPTERON QUAD CORE CPU 2358 SE 2.40GHZ 2MB L3 CACHE OS2358YAL4BGH | eBay
Overclocking is also possible on that board. Fun toy!