Just changed GTX cards with RTX.
Sold 1080TI + 1070TI + 2 x 1070.
Replaced with 2080TI + 2 x 2070S.
The 2080TI alone can do almost the work of all 4 previous cards with Optix. (RT cores “enabled”)
My reference was the link below and I can say the results are accurate within an acceptable error margin.
The benefit from RT cores vary with parameters that I can’t yet completely understand. (Apparently something to do with number of samples in the scene)
My personal suggestion would be the 2070S. If not, 2070.
Hope this helps.