I have a fifteen inch imac g4 with 800 MHZ processor speed. It has two slots of ram with 256 MB in each slot. I upgraded the factory slot to 512 MB low density ram from ebay and it doesn't work!! The system profiler says that the slot is empty. I put it back in three times and it still didn't recognize it (although it did recognize the 256 MB). I'm confused. Please help. I can't stand a sluggish comp.