Looks like a great upgrade. That CPU is still very viable for gaming. You won't get as high of framerates as pairing that card with an LGA 1155 i5 or i7, but still very playable in all games on Ultra. Don't worry about bottlenecking your graphics card - it depends more on whether your CPU is strong enough for the game you're playing.
You'll get more than double the performance! This of course also depends on what resolution you're gaming. If it's 720p I would stay with the 550ti and buy a better monitor or HDTV but if not I say go for it. You might also consider an ATI 7870 (the ones clocked @ 1ghz) if you can get them cheaper its a better buy as it performs better than the gtx660 depending on the game and vice-versa but I keep hearing about the recent AMD drivers giving their cards a nice boost in performance.