Going to be a challenge. Almost all cards made today are PCIE and not PCI. In addition even if you were to find one getting a driver that will run on windows 8 is going to be hard. I think windows 8 run 32bit drivers and almost all modern drivers are for 64bit OS.
You might find a USB device but again there are driver issues.
What your best option might be is to find a small inexpensive router that can run in client-bridge mode. Some of the so called repeater/externres can also do this. You would then use a ethernet cable to connect to your machine and there are no drivers to load.
In general you want to avoid wifi for playing online games unless you have no other option. Ethernet of course is best but you would not be here asking about wifi if that was a option. The next best is MoCA if you have coax cable in the room the router is in and your room. You could also consider powerline network. It might be a bit slower than wifi but it will have a much more stable signal. Both moca and powerline are connected via ethernet so again you don't have to do anything special with your machine.