God of War's depends on the threshold needed to cross to be considered "popular." I suppose it's popular in the same way Metroid is popular, as in, people remember it and count it among their favorites, despite not actually doing as well as they remember. So much in Metroid's case that Metroid Prime 3, despite being criticized for doing "poorly," actually outperformed Super Metroid.
IF you want actual, raw popularity, God of War ranks in behind Cooking Mama and God of War 2 ranks in behind Professor Layton. This of course, is sourced by the controversial VGChartz.
Of the games "system seller" status... eh... call me skeptical. The current status of the franchise rests on God of War 2's 2.5 million sales on the PS2 (if you don't want to count the PSP version's 1.7 million, which for the sake of artificially pumping it up, I won't) and that's about the same as the ever controversial Wii Music. To see a "system seller" in action, one has to look at Mario Kart, and how it seems to be still selling DSs and Wiis, to which each game has enjoyed record sales (with Mario Kart Wii now being the highest selling racing game of all time.) And besides I'm sure anybody who cares about God of War already has a PS3.
TL:DR: God of War may be popular among Sony fans, but that doesn't mean it's popular in general, nor does it make it a system seller.