Which GPU will provide me more value for my really old system?

Hello, I’ve been away for a while due to which I couldn’t complete my GPU upgrade. Earlier I had the 1660Ti in mind, but then considered the RX 5700 model(This was about 2 months ago). However, I’m in doubt now that the RX 5700 will provide me better fps due to the huge bottleneck from my old system. Am I better off getting a 1660Ti for now, until I can complete my other upgrades, or will be 5700 be good enough to see a noticeable fps difference. We can consider games like Borderlands 3, AC: Odyssey, Cyberpunk2077, Control, etc.

Suppose if @1080p, if a RX 5700 GPU gets 100 Fps on a really good system(Good CPU and faster RAM) for Borderlands 3, how much fps will I see while playing with my configuration. And if I replace it with 1660Ti, will the fps remain the same due to bottleneck issues?uc browser shareit appvn

My CPU/Mobo/ram upgrade won’t take place till next year, might even get extended into mid-late 2020 (By that time news on new GPUs might come out, and this current upgrade will remain useless because it was bottlenecked, and never performed to its potential).