I'm currently using an RX 580 and have about $240 to spend on a graphics card upgrade. What would offer the biggest performance improvement? My current power supply is 500W, though I'm planning to upgrade to a 650W unit.
2 Answers
A used RTX 3070 could be a strong option around $240, especially if you can buy locally and avoid shipping costs. It would be a massive upgrade over the RX 580, but make sure the card is tested and that your power supply has the required connectors. A quality 650W unit would be a safer match.
Check your power supply before choosing a card. A 500W unit may be borderline for something like an RTX 3070, depending on its quality and the rest of your system, so upgrading to a reliable 650W model first is a good idea.

That would be a huge generational jump from an RX 580.