In ear earphones will work for PC gaming but one of the problems is that you wont have surround sound, preventing you from knowing where the sounds (ex. footsteps) are coming from. Which can be very important if you like to play competitively most First person shooter games.
A separate mic would definitely work and there aer many that you can find for cheap on the internet on sites like Ebay, Craigslist or Amazon. just be sure to check out at least one review on the mic before buying it so you dont end up sounding like muffled static when you talk to your friends.
If your mic is built in, the best way to test out if its clear enough is to just ask your friends on Teamspeak, Discord or ingame voice chat if youre clear or if you have static. If they say that its good enough then youre fine using it.
Good luck, hope this helps.