Before I get my new PC I want to make sure I can use a wireless USB to connect to my router since it has a 1 - RJ-45 Ethernet Connector.
well, whats your router type (is it even wireless)?
A wireless USB what? 
If you’re talking wireless USB network adapters, generally speaking they should mostly all work. There’s 4 main wireless network types, A, B, G, N.
A and B are pretty much obsolete, chances are anything you have in your house is a G and N unless it’s fairly old. I’m not even sure if you can buy a B anymore, and I know A is long-gone. As long as your wireless USB stick is the same or below your router’s it should work. For example, if you have a Wireless-N router, and a Wireless-G USB adapter, you’ll be fine. But it does not work the other way around. The wireless adapter must be the same type or an older generation.
Also, if you have the option to, make sure to get a Wireless N. They are 10x faster than G (50Mbps~ vs 550Mbps~). Although that won’t speed up your internet, because unless you’re using broadband beyond 50Mbps you won’t notice. Chances are your internet’s speed caps are probably less than a Wireless-G router’s maximum speed. But, when it comes to transferring files it can’t be beat.
There’s a lot more to it, but these are just the basics to get you on track.
£15 gets you an 8 port gigabit switch; the number of ethernet ports on your router shouldn’t be the reason* for opting for wifi.
*mobility & convenience on the other hand are perfectly reasonable justifications for wifi.
Yeah… I live in america.
And you can’t convert euros to dollars?
Literally two seconds on Google will get you the conversion.
That’s actually 15 pounds…
Argh sorry… thanks for pointing that out, I make that mistake a lot.