sli 9800gx2's loose the a single gx2 by itself in some cases. and in alot of games 9800gtx in sli barley makes a difference eg. ut3. There are some games like bioshock where sli 9800gtx's does perform better and close to 60%. at higher resolutions and increasing AA it does help to have sli but sli 9800gtx's is stupid when u could just buy one gx2.
the other thing is being left with two cards when a new card comes out that will do better then ur new sli.
imo video cards out date, but sli out dates quicker.