Bottlenecking is when the rest of the system can deliver much more power, but is held back by one component. The term comes from pouring liquid out of a bottle - the narrower the neck, the slower the flow rate you can reach.
A Phenom II x4 965 and a Radeon HD 6870 is an excellent combination for gaming. That CPU will in no way hold your video card back. If you had that video card along with a 2.4 GHz Athlon x2, the Athlon would be a bottle neck.
In general, any processor with 3 or more cores has enough capability to handle any video card with little to any bottlenecking.