what is the max memory for an early 2008 iMac

The iMac (Early 2008) computer has two SDRAM slots in the bottom of the computer and comes with at least 1 GB DDR2 SDRAM installed into one slot. The maximum amount of RAM you can install in the computer is 4 GB (a 2 GB SO-DIMM in each slot).

Source:

iMac (Early 2008): Memory specifications - Apple Support

I believe the official Apple answer to your question is as stated above--2g per slot. However, I upgraded my early 2008 iMac with the Other World Computing upgrade consisting of a 2g in one slot and a 4g in the second. The memory is recognized by my computer and the new setup works just fine.