What’s the best budget upgrade path for an i5-8600K and GTX 1070 Ti?

Before replacing everything, check temperatures, background processes, game settings, and whether the CPU is running at its intended boost clocks. Updating the motherboard BIOS and moving from 16GB to 32GB can also help with modern titles and multitasking, although neither will transform graphics performance. A used i7-9700K may provide a modest improvement on this board, but only buy one if it is clearly cheaper than moving to a newer CPU and motherboard.

If the budget is very tight, prioritize the graphics card only if your games are GPU-limited at your target resolution. A used RTX 3060 Ti, RTX 3070, RX 6650 XT, or similar would be a noticeable step up from the 1070 Ti, and the 750W power supply is more than adequate for those cards. However, the 8600K can still cause stutter or lower minimum frame rates in CPU-heavy games, so a GPU upgrade is not a complete solution. For a balanced upgrade, save toward a newer six-core CPU and motherboard, then upgrade the GPU afterward.

The fact that lowering graphics settings doesn’t help much points toward the six-core i5-8600K becoming a CPU limit, especially in newer games. I’d start with a platform upgrade and reuse the case, PSU, cooler, and DDR4 if you choose a compatible board. A Ryzen 5 5600 or Core i5-12400F-class system is a sensible budget target. Then replace the 1070 Ti later with a used RTX 3060 Ti, RTX 2080-class card, or Radeon RX 6600 XT and above. Check actual game performance and prices locally, since the best value depends heavily on the used market.