Very few folks would opt for socket 2066 over latest socket 1151 with a Z370/Z390 and an 8700K or 9700K, with only a few opting to spend perhaps another $150-$200 more on a 9900K...
Unfortunately, you current mainboard would be maxed out w/ a 7700K, with those scarcely available except in used market and at elevated prices.
But as the 6600K will limit you in latest games, I'd probably start there, so upgrade CPU and MB (Z390 and 9700K) and try to add a GTX1660 (or better yet, GTX2060!) later...(the reviews of the 1660 have it pretty close to GTX1070-level in performance...)