Your CPU is still good, I doubt it would be a significant performance bottleneck to any GPU currently available. In other words, just buy the best GPU you can afford, of course depending on your performance requirements. If you were considering a GTX 1080ti, you might as well buy an RTX 2080. Slightly better performance, more future proof, and some fancy new features. The RTX 2080ti is overkill ( and overpriced ) imho, unless you absolutely want to have the best gaming GPU.