Frame rate drops when opening applications.

• This might be because your i7 is showing its age. Not entirely your gfx (or GPU) card. I suggest you clean your case and fans for dust and try cleaning the old thermal paste on the CPU as well.

For compatibility with your Power Supply and PCI-E slot.

• There's not much to worry about here. You are rocking a GTX 980 so I would assume that your current PC has a Power Supply of at least 450 watts. Now, you want an Intel Arc A310. Don't do that. I mean, you can but I suggest you don't do so for the simple reason that you are replacing a 4 GB card with another 4 GB card. Also the PCI-E slot will be just fine. My concern would shift to the PC case that you currently use (the new GPU card might be too big for it).

Compatible with Linux, Firefox, and Wayland, with lots of hardware codecs. Intel ARC?

• You can go with Arc B580 (12 GB) if you want to go with the Intel route or
• AMD RX 9060 XT (16 GB).
• Both are compatible with Linux and mostly share the same dimensions with your GTX 980.
• Stay away from Nvidia cards for Linux compatibility.

How do you learn all this stuff?

• Let's just say I had my fair share of making a smoke machine with my PC.
• Try looking up videos on PC building guide. They usually explain things like compatibility, what do these numbers mean, etc.

Why go nuts with Arc B580 or RX 9060 XT?

• Why not? These are fairly recent GPUs and should handle most of your needs daily driver-wise or gaming-wise. I wouldn't worry much about your processor it will still do its job. And having a much bigger VRAM with your GPU is always always a priority over lanes or interfaces (entirely different issue - story for another time) or even your CPU.