Every time you plug something in to your computer you use resources, whether it's a usb adaptor or not, that should be obvious. The same is true if you install a wireless network card instead. However, the difference in performance from either method will hardly be noticeable.
A wirelss signal, via usb adaptor or internal network card is subject to variables that make it less than 100% reliable, hence the reason to keep your main computer wired.
The difference between 54mbs and 108mbs should be noticeable and should equate to an increase in throughput speed.
Go for 11g every time, the spec is superior to 11b. 11b is more or less oudated now.