@AphoticLee
You can opt in either, as they are both around the same performance with the GTX 1660 and the RX 5500 XT. The GTX 1650 is more budget-friendly and consumes less power (but is weaker).
However, the Radeon RX 5500 XT has more VRAM, which is helpful towards other games that is VRAM-heavy. You'll be fine with 4GB VRAM, but for more future-proofing or other games, 8GB VRAM would be the better choice.
All in all, I would opt in the GPU, DDR3 16GB RAM, and a better power supply.
EDIT: In regards to building it piece by piece, it is usually only worth it when it comes to RAM, GPU, adding in a new hard drive or SSD, and sometimes power supply. Your CPU will be mostly fine for the Sims 4 (but you'll need a new CPU and motherboard for when the Sims 5 rolls around).
EDIT #2: As a side note, speaking of HDDs/SSDs, purchasing an SSD will greatly help in faster load times on the Sims 4 (as well as boot times via Windows, but you'll have to reinstall Windows on that drive). A 1TB Samsung EVO is currently on sale via Newegg.
https://www.newegg.com/samsung-860-evo-series-1tb/p/N82E16820147673
The Western Digital Blue SSD is cheaper (but the Samsung EVO is superior).
https://www.newegg.com/western-digital-blue-1tb/p/N82E16820250088