What CPU upgrade makes sense for my i5-9400F and RX 6750 XT?

The best option depends heavily on your budget and motherboard model. If you want to keep DDR4, a new platform such as an i5-12400F or 14400F with a B760 DDR4 board, or a Ryzen 5 5600 with a B550 board, would be a noticeable improvement. A Ryzen 5 5600X is only slightly faster than the regular 5600. Higher-end chips such as the Ryzen 7 5700X or Intel i5-12600K/14600K should be paired with a decent aftermarket cooler. A fresh Windows installation is usually a good idea after replacing the motherboard.

Before buying anything, check the exact motherboard model and BIOS support. If it supports it, an i7-9700F could be a drop-in upgrade, but the price often makes that a poor value compared with moving to a newer CPU and motherboard. A full platform upgrade to something like a Ryzen 7 7700 would offer a much stronger long-term path, but it also requires DDR5 memory and costs more.

27GB is an unusual memory total and may mean the sticks are mismatched or running in an inefficient configuration. Ideally, use a matched 2x8GB or 2x16GB kit so dual-channel operation works properly. Adding a fourth stick is not automatically better, especially if the motherboard slot is faulty or the modules have different speeds and timings. Check the motherboard manual and test each slot and stick individually. Also consider adding an SSD if the system still uses a hard drive, since modern games benefit significantly from one.