The key here is how fast you intend to upgrade the rest. If you're doing that within one year, by all means go big(ger) on the GPU. If not, keep it cheap, maybe even second hand.
Its not a good idea to buy a GPU that you cannot fully utilize, especially with new releases right in front of us. After all, a new release will push the price per FPS down a bit, by as much as 20-30%. Better off jumping in by then and get more mileage for your money, and get something that balances out nicely with what you have today instead.
On a 3570 I would just start saving for the whole rig minus perhaps storage, case and PSU. Get a good picture of what balance you are looking to achieve on a new platform, and scale your GPU on that.
Another thing of note. A 1660S or ti will not be having RT capability, but there will be 2020 GPU releases from both camps that dó have those resources, also on a similar price point as the 1660S is now. The resale value of a 16 series GPU is just about zero by the time you might sell it.
Just for an impression on what you might miss out on... a 1660S is a little bit slower than my 1080. Here is what I got going from a 3570K at 4.2 Ghz to a new CPU. This is 1080p, too. Note that FPS is capped at 120. But you can see the 3570K might get close at times, but it is brutally inconsistent.
3570K
8700K