A GTX 660 TI and a GTX 550 TI have the same memory bus type and width. The 550 TIs would have far worse GPU performance, but far higher memory bandwidth. I think that when overclocking is involved, the 550 TIs might win, but otherwise, they'd need some AA, tessellation, or advanced lighting features activated to win because the 660 TI has much lower efficiency with those common features.
Either way, the 550 TIs would probably have some noticeable micro-stutter, so it would be a trade-off, at best. If you buy a whole new card, I'd recommend a 7870 or a 7950 because they would also cope with AA, tessellation, and advanced lighting features (among others) better than a 660 TI.
However, that's not to say that the 660 TI couldn't make you happy that you bought it, maybe it could, but I don't consider it the best solution nor the best solution for the money. If you must buy Nvidia, then I recommend at least saving up and going for the GTX 670 rather than the 660 TI. It alleviates some of the issues (especially the AA issue).
The 660 TI, 7870, and 7950 would also probably all draw less power than two GTX 550 TIs, espeically the 660 TI and 7870, although the 7950 is not far behind them. The 670 draws about the same amount of power as the 7950 does, just in case you want to know.