Z370A-PRO
Z370A-PRO
The top CPU choice for that board is a 9900K, but LGA 1151v2 chips are usually priced higher than their actual value in the used market. It’s more practical to purchase a fresh motherboard and AM4 components instead of risking the board. You might be able to get it sold for around $60 to $100 on eBay right now.
You aim to improve performance while staying within budget. Current options like i5-8400 or i7-8700 feel outdated today, but they still work well for gaming on older titles. Pairing them with a mid-range GPU such as the RX 6600 gives solid 1080p results for a couple of years ahead.
For a budget upgrade we used 50$ per 3600(x)/80$ 5600(x). It cost around 50$ per month or about 450 in a year. If you want something future-proof, consider an upgrade to a board with at least the B650M HDV and a 6000C32/30 DDR5. Coffee Lake is only moderately appealing because of its IMC, but Comet Lake still offers a better IMC. Cezanne (1:1 DDR4 ~5000) is another option.