Alienware X51 R2 Upgrade

Two things to consider - power draw and physical size.  The 750 TI in most forms is a single slot half length card with a TDP of only 75W (which it can get through the PCIE slot) and doesn't require any additional power connectors.  The 1660 - whilst many are half length most seem to be dual slots (so are twice as thick as the 750Ti) and it is also a 120W card so requires at least an additional 6 pin power plug.

If you want something that should just be a drop in upgrade then your are really looking for a single slot 1650 e.g. https://www.gameshedge.com/gtx-1650-gainward-palit/ 

If you want more of an upgrade the have a look inside your R2 the PCIE slot has a riser card and a metal shroud for the GPU and should be able to fit a dual slot card in there.  There should also be a six pin power connector from the PSU - (https://www.manualslib.com/manual/1064273/Alienware-X51.html?page=36#manual) so  you should be good to go with the GTX 1660 (depending on the physical size).