Ram upgrade on my imac 20" early 09/08

Hi, Tried to oppgrade my Ram on my imac 20" early09or 08. Here i can uppgrade the fre slot for memory, but the primrary memory in the first slot, i cant change. When i try to do so, My Imac won't start up, it only gets a black screen. I've tried to use both of my memory cards in the free slot, both of them works fine. The only thing i can guess it has to be, is that my mac cant have more than 3gb ram or something?

solutions any1?

Imac
20"

2,4 Ghz intel core 2 Duo

3GB 667MHz DDR2 SDRAM

Ati RadeonGD 2600 Pro 256MB

now i have Slot1:1Gb orginal memory Slot2:2Gb wich i changed(works fine)