What's the best graphics card thats worth getting to upgrade an older PC?

My PC is getting on a little, it's running on an i7-4790k, 16gb ddr4, and a GTX 970. right now, it's struggling some in most games even though I don't play any triple A titles. What would be a sensible upgrade that wouldn't get totally bottlenecked by the CPU?

In most games I'm playing the CPU is pinned at around 25% while my card is maxed at 100% 3d and vmem usage so I'm fairly sure it's just my card that's the main limiting factor.