do you think a T8700 would work?

i just opened this thing up and its shocking how easy access is the cpu is.

Here is exact specs of this iMac,

http://www.everymac.com/systems/apple/imac/specs/imac-core-2-duo-1.83-17-inch-specs.html

i am just curious what is the highest i should go?

any info would be appreciated!~