Others have commented on performance, so I don't have anything to add. The parts/cost is really large and varied topic depending on what you're trying to do and what your budget is. And pretty much every part of my rig has gone up in cost since I got it, so no idea what it would cost to build at this point, but more than I'd be willing to spend.

So I'll just cover a few things you may run into issues with when trying to game on Linux.

Anti-cheat: Some anti-cheat software is not compatible with Linux or the developers didn't properly set them up for Linux. There isn't anything you can do to work around that. Just the way it is.

Some launchers/DRM: Ubisoft and EA seem to be big ones for launchers that are broken regularly. There may be others as well, but those are the ones that come to mind. You can sometimes get them to work by switching to different versions of Proton, but it can be hit or miss.

Niche gaming hardware: Some niche hardware like steering wheels, shifters, pedals, flight sticks may have some issues. Some work right out of the box, some work but can't have software/drivers updated, some can't be customized because of proprietary software that is windows exclusive. This hobbies have some pretty dedicated fans, and more of them are making the transition to Linux. So you might be able to find assistance here on Lemmy or on various forums if this is something you're trying to get working.

You may run into other minor issues but there are a ton of resources out there to get games working. Most will rum pretty much like they are native. I think Linux is a fantastic platform for gaming, and it will likely continue to be better as more people adopt it.