Freezing, glitches and weird graphics problems

My white iMac starting having weird praphics issues about year and a half ago. Nothing that was too much to deal with. Simply using expose always cleared it up and if it came back I would do the same. It seems to be getting worse. And then yesterday my iMac started getting super slow and pausing for no reason and then I got the black transparent window shade come from the top of my screen to the bottom and then a message telling me I needed to restart my computer.

After that seemed to work but I still had graphics problems screen flickering and things like this. Then freezes started happening. I reset my pram and novram with no luck. That didn't seem to help. I have 4g of memory so I know it's not that. What hardware could be causing this? Thanks

Btw using the latest version of snow leopard