We're defining "system seller" as a game which provokes the consumer to buy a game right , if we're not specifying any specific quantaties then i'm certain that God Of War III will be a system seller of sorts , maybe not a very big one but people will buy a PS3 purely because of god of war.
There are some reasons why God of War might sell systems.
1.Fans of the God Of War might series might feel it's the right time to jump on to the PS3 because of god of war.
2.Sony will most likely heavily market & promote the god of war franchise.
3.God Of War will most likely be bundled at the same cost of a non-bundled PS3 , this will be an incentive also.
But there's also some reasons why God of War might not sell systems.
As the generation progresses the libary of games becomes larger and more diverse, this makes it harder for individual games to have a substantial impact , the obvious exceptions to this is games with massive brand name in relativley niche genre ( Metal Gear Solid 4 , Gran Turismo 5 ) , they offer incentive as individual games because there's not alot like them the market that can offer a similar game experience.