Read this article on best graphics cards for the money:http://www.tomshardware.com/reviews/gaming-graphics-card-review,3107.html
Bottleneck is not a good term. It implies that a graphics card can cause less performance.
What you are asking is at what point do I get diminishing returns for the incremental dollar.
The graphics card will not cause a bottleneck, regardless of how strong it is.
Stronger cards give you better average fps, higher minimum fps, and more eye candy.
Buy the strongest card your budget and psu will allow.
If your budget is $300, look at the newly launched GTX660ti.