Hello and welcome to the forums
According to http://www.asus.com/Product.aspx?P_ID=KyHOsOKWujC2QguJ&content=specifications
It supports all dual core 1333MHz FSB CPUs so it should handle Q8300 just fine.
I also found these links which confirms that it works http://www.overclockers.com/forums/showthread.php?t=633783http://www.tomshardware.com/forum/253833-30-q9550
"I have installed Q9550 on P5N-E SLI after bios update to version 1301 and everything works fine (also processor speed at 2.83 GHz).
I didn't try to overclock cpu."
"I just put a Q955O ON MY P5N-e SLI with the 1403 Bios. It works fine but the core voltage is at 1.403 and I can't overclock anything, including ram. The CPU seems to run a little hot. Don't know if I need new motherboard."