The 550W (its modular too) will power most single cards I think you could realistically be wanting. Such as,
ATI 6950/GTX560ti or anything cheaper/slower.
The 650W will do crossfire/sli (two cards) of the 6950/GTX560ti and anything slower in pairs.
ATI 6950/GTX560ti or anything cheaper/slower.
The 650W will do crossfire/sli (two cards) of the 6950/GTX560ti and anything slower in pairs.