You could either get a wireless usb adapter or a networking card (which uses a pci or pcie x1 slot). They'll both have pretty much the same performance, whichever you get is just based on preference.
If you have a router that supports 5ghz and 2.4ghz frequencies, I'd get a dual band card/adapter.
Stick to brands you know, don't get something from a company you've never heard of and check how easy stuff is to install (software wise).
In the past I've bought netgear and asus adapters, and I found that both were terrible to install the software for.
I'd recommend the asus n53. Even though it took a while to get 802.11n working, if you don't install the asus software and instead use the windows utilities, it works pretty well. Also, getting the 802.11n working was mainly due to location (I had to tweak some obscure settings to get it to work using the right channels for the uk).
TL;DR: I'd recommend the asus n53 or single band equivalent if your router doesn't support dual band. Bear in mind that most adapters or cards will be a pain to setup though.