I've seen computers with built in wifi and I was wondering if I could buy the kit to do it. I know of one way to do it; and that is buying a PCI-E card that is a converter for a mini PCI-E and put a laptop wireless card in there. Is there any other way? and what would be the fastest?
Not the best way to do that unless you just absolutely have to have it in the case.
Best option is a $10 USB wireless card.
Next option is to use a ethernet to wireless bridge, many times they are sold as "gaming adapters" because they are mostly used for non wireless xbox. These tend to have multi antenna configurations that allow full mimo if you need that feature