Won't you spend most of your gaming time plugged into the power grid? If so the 6 vs 9 cell battery won't be a factor.
Gaming on your battery? It won't last nearly as long as you'd hope, Im sure.
It looks like there is a MSI 9 cell battery available based on this webpage.http://www.msi.com/index.php?func=prodnbspec&maincat_no=135&cat2_no=&cat3_no=&prod_no=2127
You might be able to special order the 9 cell if it's critical to your gaming experiance.
Otherwise, maybe a 2nd 6 cell battery?