Hi, I'm building an expensive new PC which has a Asus z370-e gaming motherboard. It has a built in Wifi, but I want to make sure my connection is perfect. The one good one I've found is the Asus PCE-AC88.
Money is better spent on a really good WiFi router than a different PCI WiFi card, I've had a few Asus boards and Asus WiFi cards in the past and they worked about the same, what makes a connection really good is low obstruction (less walls/object between PC and router), signal strength (make sure to check local WiFi noise with something like WiFi sonar, set your channel to an empty band), and making sure no metal objects are near the router or PC (also do not place them directly against the wall, pull them out as much as possible. Also having a good ISP helps but that's usually not something you can change because of local monopolies.
Might want to search google for "best routers" and find the best one in your budget. Review sites like Tom's usually have roundups where they compare some models.