graphics bug after install Mountainlion

[nycco]

hello
My name is Nicolas, and I'm glad I managed to install on my pc MountainLion thanks to all your tutorials but I have graphic bug in particular when I open the notification center or when I stay long on Safari.
See image

I have this motherboard http://www.gigabyte.fr/products/product-page.aspx?pid=2615
with an ATI graphics card HD 3800 512MB readom is recognized in "About This Mac" But with 5mo instead of 512 ...
I wanted to know if there is a solution a kext?
Thank you in advance

 

[nycco]

please someone already had this problem?
thanks