I own zyxel 500mbps units I got as a pair for 70 bux at buy.com and they are streaming blurays and hd-dvds from my downstairs home server to my living room tv well, as well as sending my over the air tv signal from upstairs to my downstairs computer using an hdhomerun.
If you are paying rent for your current cable box, you may want to send it back, *after* you have determined that your new setup is working flawlessly. Because you could install the nettop on any tv in the house, and you could use your xbox360 as a media center extender to get that identical nettop media center feed to the other tv. The xbox 360 is designed to be streamed over N wireless and I think it's very possible you wouldn't need to buy a 3rd powerline network unit, and could use your existing N network with your 360, but you'd need a N adapter for your xbox 360 which you may already have. If you don't have one and If the price of the n adapter is close to a powerline adapter then use a powerline and plug the ethernet into the 360 as wired is almost always better than wireless.
Another idea would be to setup the hdhomerun box in place of your current cable box on the main floor near your tv, and install the nettop pc also on this tv set. Possibly, you could wirelessly stream this to your downstairs xbox 360 extender and avoid having to purchase any powerline adapters, because the signal to the hdhomerun would be fine (since it's coax) and media center and the 360 convert the stream into something that travels better over wireless. The raw unconverted hdhomerun feed is touchy and does not stream well over wireless but it shouldn't be a problem in the scenario just described. However, I must say I've never installed such a system but in theory it may well work, and you could try it first *before* you shell out for the powerline adapters.
Something like this: http://www.newegg.com/Product/Produc...82E16883103297
or this: http://www.newegg.com/Product/Produc...82E16883108536
and this: http://www.newegg.com/Product/Produc...82E16815345006
plus two remotes, one for the nettop tv and one for the 360 media center extender tv (xbox doesn't require IR usb dongle as it has IR): http://www.newegg.com/Product/Produc...82E16880101002
You could try this hardware first, and if it fails to stream flawlessly, then get three powerline adapters (one for router, one for nettop, one for 360) and set all of this stuff up as I originally suggested (hdhomerun into router etc). If your router is very close to your upstairs tv near the nettop, then you would only need two powerline adapters and not three as the nettop could plug directly into the router, and you would only need an ethernet out cord from the router to the powerline ac unit, and a powerline unit downstairs for the 360.
Also keep in mind that you'll get the best price if you go to slickdeals.net and shop for best prices on components over the course of a month or two. You could easily save 100 bucks or more over buying all components from newegg.com right this moment. eBay can also be your friend in acquiring these components at a discount from retail.