PCIe 2.0 isn't a problem. Your older system is. The GTX 460 is a good GPU. The 660 ti will be bottlenecked by that CPU. Save for a new system. You could certainly buy the 660ti now, the thing will still work, but since you have a capable 460, I would wait till you have enough for a new system and end up getting something better.
+1 to that. It's exactly what I say to the bottlenecking questions - simply means when the time comes for a CPU upgrade, you unlock the rest of your GPU potential and get a huge boost!
i don't fully agree. it depends when/if you plan on updating your cpu. if you have no plans of getting a new cpu anytime soon then buying a gpu that will be bottlenecked makes no sense. by the time you actually do upgrade the cpu you will probably want to upgrade gpu anyway. save the money and get a gpu that wont be bottlenecked. unless you are going to upgrade your cpu soon.