It's true, Nvidia is massively more powerful than AMD at present, no comparison at all.
However, their DX12 support is lackluster, and you can get an 8GB card for under 300 that performs extremely well for it's price. The 6 GB GTX 1060 performs better in benchmark software, but anything truly utilizing the parallelism allowed by DX12 going forward is going to be wasted on it's power. It's also 2GB shy, and that makes a big difference when you get a game that can use that extra ram. In real world testing of actual video games, they're largely even, and the rx 480 seems to be in the lead typically.
If I wanted to blow $700 on a video card that would be antiquated two years from now, , sure, there's no contest, it's a 1080 GTX or nothing. If I don't, and there's no reason to on a 5 year old system that will need replaced soon, there's no reason to go Nvidia when their hardware is an evolutionary stage behind the competition when it comes to parallel processing and will probably perform quite poorly on anything that comes out a few years from now. That system would be CPU bound in most games under DX11 and lose a huge amount of performance versus what the 1080 is actually capable of, it's a terrible buy.
They've always had good budget cards, and for a PC that old, you really shouldn't buy anything else even if you have money to throw away. This isn't coming from some Radeon fanboy either. Whilst pining for the golden age of Voodoo cards, every system I've built myself has been run by a high end Geforce. I don't see much hope of that continuing though, they're radically overpriced, and not the least bit future proofed.