Is an RTX 5070 a worthwhile upgrade with an i5-8400 for 1440p gaming?

A GPU upgrade won’t make the CPU overheat. The cooler question mainly matters if you replace the i5 with a higher-power CPU, since those chips can produce more heat than the stock cooler was designed to handle. With the current i5-8400, the stock cooler is generally fine as long as temperatures are normal. Also, newer games do support the RTX 5070; the card is simply much faster than your current GTX 1660.

Buying the 5070 now can make sense if you want to improve your gaming immediately and already plan to replace the CPU later. The main downside is that some games may show inconsistent frame times or lower-than-expected performance because of the six-core, older CPU. If you can tolerate that temporarily, the GPU can move into your next system. Otherwise, saving for the CPU, motherboard, and memory together will give you a more balanced upgrade.

For 1440p at a 60 FPS target, the i5-8400 should be usable with an RTX 5070 in many games. At that resolution, the GPU will often be the limiting component, especially if you use higher settings. You may see CPU limits in large open-world games, simulation titles, or games that demand very high minimum FPS, but it shouldn’t make the card useless. Just make sure your power supply is adequate and has the required connectors.

You could look for a used, compatible upgrade such as an i7-8700K, i7-9700K, or i9-9900K, depending on your motherboard and BIOS support. However, older CPUs on that socket can be overpriced, so compare their cost with a newer CPU, motherboard, and 32 GB of memory. Don’t automatically spend a lot on a used 9900K just to avoid changing platforms.