My graphics card covers my PCI-E x1 slot

If you have an open x4, x8, or x16 pci-e slot you can plug the card into that port and it will work just fine.

As far as USB adapters being slow, they are not as good as pci cards.
Now for browsing the web you wont really notice, but for gaming or transfering large amounts of data over local network then yes you will see a speed drop.

If using for gaming or lan file transfers then you will be better of using wired instead of wireless. If you cant run an ethernet cable to the computer then the next best thing would be to use a powerline network adapter. A decent 500 or 600mbps model should run you $40-50. The powerline adapters will have a better response time for gaming and will transfer data faster then most wireless N setups as well.