Seems the original question keeps getting lost on folks here, as he was asking for hardwired to wifi (or other) adapters, not USB or PCI wifi cards. I've used a few options.
The first (and easiest) option (which eudimorphodon mentioned) is using a powerline adapter. However, as he's mentioned, your success will vary with those depending on your house wiring. I almost inevitably had issues with the old '200' models, but the 'AV600' models or better are much more successful and usually only a few bucks more. I wouldn't recommend them for for your main system that's watching youtube, but for a retro system they're just fine. You'll need to have at least two, one connected up to the router and the other connected to the machine (or hub, if you have multiple machines in the area).
Second option, and this is if you have cable run throughout your house, is to use MOCA or DECA adapters. One end would plug into a router and your coax cable, and the other end just plugs into the coax somewhere else in the house. I actually use a number of DECA adapters around my house because I don't have any cable or dish tv at all, so the coax lines were unused. If you do have cable tv or dish, then you have to use one or the other depending. (moca interferes with directtv, deca interferes with cable tv... so you need moca with cable or deca with directv). I use the deca adapters, they sell them on amazon for about $15 / pair.. save yourself some grief and get the wired ones, not usb. I have a 100 Mbps connection and get about ~80 on these, which of course is more than enough.
The third option (which you alluded to in the original post) is the RJ45 to wifi adapters. These are actually pretty handy because you can do multiple things with them. The one I have does at least 3 things. It'll convert a wired connection to wifi (good for hotels that either don't have wifi or have annoying requirements about max devices), wifi to RJ45 (which is what you want to do), or wifi to wifi (again, good for hotels to circumvent the 3 device limit or avoid reconfiguring your devices every time you travel to a new hotel). I can't specifically recommend one because the one i use is actually a portable unit that is also a battery (Ravpower RP-WD03), though you technically can leave it connected to power but it might be a bit much for what you need. All you do on these is connect to the device via a network cable or wifi, go to it like you would a router (usually 192.168.1.1) and configure the settings for the method you want, save the settings, and hook it up to whatever system you want.
Finally, the forth option. This is basically what the 3rd option is as well, technically. But, if you have an old router that works in 'client bridge' mode, or supports something like DD-WRT, you can reuse that. You basically just set it up, have it connect to your existing wifi and 'bridge' the network ports on it to your wifi.. This works great, has great signal, and probably will be free because you might have old routers sitting around your house that can do this (or pick up one from the goodwill). As mentioned, one that supports DD-WRT would be the easiest, but some good routers will also have this mode as well (I think asus in particular). Google 'wifi client bridge'
Let me know if you have more specific questions on one of these.