I would recommend getting a router over a USB dongle. For one, a router doesn't require the PC to be on to give your other devices wireless access. Also, if you ever decide to ditch your desktop and just go with a laptop, you aren't going to want to be tethered to your modem plus have that dongle on all the time.
Just go buy whichever wireless router on Amazon has the best reviews. You don't need to buy a WiFi card for your PC. You can just plug it into the router. All wireless routers also have a few wired ports as well. Plug your cable modem into the router and BAM, you have WiFi. Setting up security and everything shouldn't be too hard, most routers these days come with simple setup options, so you really just have to follow a few on-screen prompts and you should be set.