A USB one is a little simpler since you do not have to open the case up or worry about which slots you have. The internal cards work about the same as USB.
Your router is a older model and does not support a lot of the new features. Still I would not buy a old USB card to match but it does not do a lot of good to spend a real lot until you are thinking about replacing your router. Any 801.11n device will be more than good enough around $20-$25 will get you something that will work.