My Asus A8N-SLI motherboard? and my 500 watt Power Supply Unit? Just asking for the simple reason of, i have the 7800GT in there at the mo, and would like to upgrade to the 8800GT, but ive heard that some asus motherboards are not compatible... can anyone please help??

It looks like it should be. But it will be held back by the rest of your system. Your motherboard tells alot about your setup =D. But to answer your question, from what I can see, yes. But like i said, your motherboard supports only Socket 939 processors, and only DDR-400 Memory.