Wireless cards only use an x1 slot so x4 would work fine.
There is really no difference in performance (PCIe v. USB) if you account for the antenna size, and the ability to move the antenna around can really help, so something like
THIS for 2.4GHz.
If you have a lot of other 2.4GHz networks around you consider 5GHz or a pair of good powerline adapters. The new Zyxel PLA5205kit adapters are really good for high bandwidth and low latency uses like video streaming and gaming compared to any other powerline adapters that I have used.