You should start off by checking out your router. What type of router is it and what kind of wireless adapter is compatible. If the router is a Wireless G class router (54Mbps) it would be best to buy a G class adapter.
If it is Wireless N class (150 - 300Mbps) you should look into an N class adapter to utilize it's full potential (range and speed).
Not to say that an N class adapter will not work with a G class router but you will probably end up paying twice the cost and never enjoy the full specs it is offering.
As for the USB/PCI option, I personally prefer an adapter that can be placed up on my desk and not behind my PC case under my desk. Either USB or PCI it doesn't really make a difference but it is best to have the option to position the adapter for the best signal reception possible.