There is a decent chance the 580 will still have some factory warranty left on it. The 480, not so much. Most go by date of manufacture for warranties. With two to three years of coverage.
I'm not aware of any compatibility limitations with a 480. They are essentially the same card. Overclocking should bring it on par. It looks like some people also flash it to a 580.
The 8GB vram can be useful in a few tasks. Such as games, some video editing functions, GPU compute tasks, &c.
As I recall. Certain RX 580 models can be flashed with a Mac ROM. Allowing boot screen support.
If you have another card by it. You should get a card with a reference cooler. They use a blower, vent heat out the rear and generate high static pressure. Recirculating coolers need air space to work efficiently. The trade off with blowers is they are louder.