The lowest level card anyone should be getting is the 2060 or 2070.
I wouldn't be dropping serious coin on a Titan X or Titan XP again...
It's about the FUTURE!
Simply pointing out that you shouldn't be getting a 2060 and assume you'll be benefiting from RTX ... because you won't. Even the 2070 and (depending on resolution) the 2080(ti)) struggle to implement ray tracing at appropriate fps (on the limited titles available). With respect to the "lowest level" card ... that would be based on budget as a 1660ti performs on par with a 980ti/1070ti and a bit slower than a 2060. However, that is a very good option for folks on a budget. Anyway, get a 2060 because it falls within your budget, because it is a strong card and at a fairly good price ... not because of RTX.
RTX in general ... I would say give it another generation or two ... and a few more years for developers. That performance hit right now ....