Your current setup should be able to handle it with a new GPU. If you want to spend $300-$400 bucks on an upgrade, then you'll need the works : motherboard, RAM, CPU and GPU. A B450 with a Ryzen 3600 and some RAM is the most cost-effective and forward-looking solution, but you wouldn't have any budget left for a proper GPU. And in that case, cheaper would be no better than what you already have. You could try getting a better CPU for your current platform, but it wouldn't help that much (Haswell i7 don't bring much over i5, and you'd only get actual results with a massive overclock) and would be a dead-end anyway.