My personal take on the subject (as for now) :
GeoW 2 >> LBP > Fable 2 > R 2.
But things could really change for Fable 2 and R2 depending on reviews...
GeoW 2 : a lot of hype. Will sell VERY well. I don't know what kind of reviews this game will get ("only" good because this title will be an experience way too similar to the 1st one? very good ? Exceptionnal ?). Unlikely to be a system-seller.
LBP : a lot of hype (the beta cimented it deserved it). Will sell also VERY well (just look how well the lego series sell on all the platforms). The reviews will probably be oustanding. Yeah, clearly a system-seller.
Fable 2 and R 2 : a tougher gess for these two games. Will depend A LOT on the reviews...
- Fable 2 : Clearly, this game is anticipated by a lot of gamers. But, I don't know what kind of reviews it will gather (great, but too much flaws and way too short ?). Probably very good sales too. Has the potential of being a system-seller.
- R 2 : Not a lot of hype behind it (mainly due to the poor communication from Insomniac and Sony... And the way Insomniac Games work on their titles), overlooked by many people. BUT, it could be one of the few 2008's sleeper hit. Some recent previews indeed suggesting this game could be a great one. Sales will depend A LOT on these reviews. A system-seller ? I don't think so, unless it turns out to be another exceptionnal game everyone should get.