iMac Graphics Issues?

Okay, so I run a 20" iMac and a 24" Acer dual display, it all works great and has been for the last 5 months, untill recently I have been experiencing graphics issues, so essentially its all working fine and all of a sudden when you move the mouse, change spaces or whatever both to screens will just 'flip out' and have colours flashing about both monitors in triangular shapes (I will attach a screen shot I took).

So this will happen and the only way to get it of is to change spaces a few times then it will return back to its normal state, I restart the iMac when this happens and then appears to work fine.  However I may know the reason behind this, which would be; when I upgraded to 4GB of RAM a few weeks ago. I just thought I'd share my 'experience' with you; one to just let you know and if any of you have had the same experience and how they went about it. I will contact Apple to see what they can do and get back to you.

To see my setup click here.

- Henry

Here is what the displays look like when the 'graphics issue' occurs; to the right is the 20" iMac monitor, to the left is the 22" Acer monitor.