Bottlenecks don't work the way your question implies. With any mainstream CPU, the better the GPU, the better the performance. Some games may be more limited by your CPU than the GPU and some won't be. A low end CPU may hurt performance, but it won't keep a better GPU from increasing performance.
Your GPU choice should be determined by budget or performance target. Either get the best GPU you can afford, get the GPU you think offers the best performance for the price, or get the cheapest GPU that offers the minimum performance you require. I recommend checking out this article:
http://www.tomshardware.com/reviews/best-gpus,4380.html#p1