I'm not with Laminar Research, just a fellow simmer in the UK.
Whilst I am a qualified computer technician and build my own desktop PC's, the answer to your question has be considered in two ways:-
1. Will your hardware spec's as stated above, to include a 1660ti graphics card, be sufficient to run X-Plane 11?
I would say yes to this.
2. Will your proposed 1660ti graphics card run successfully in the HP Envy 750-229 desktop?
In my personal opinion, as long as the 1660ti card is a single fan version, I see no reason why it shouldn't run OK. The "single fan" option, is important because of your PC's small motherboard format of uATX, which is only 240mm x 240mm in dimension.
On this point, the Nvidia GeForce GTX745 card that you have already is 145mm (5.7") long and so is the GTX1660ti card, so physical size should not be an issue. I assume also that the GTX1660ti card sits in a PCI Express x 16 expansion slot, which the GTX745 does already?
The power supply rating for the GTX1660ti is a recommended 450W, so your upgraded PSU should easily cover that. The 16GB RAM memory seems fine.
The only item that I cannot comment on is the CPU that you have installed? It appears from the HP website that the HP Envy 750-229 desktop could ship with one of a number of CPU type versions and some of these run at less than the recommended 3GHz!
For this reason and because my comments above only represent my personal opinion, you really should contact HP directly before committing. You may even need to contact Nvidia also for their support, assuming that it is one of their GTX cards your buying?
Hope this helps