Well keep in mind that the more wireless devices you have the more it slows down your wireless. Remember when you had a printer hooked to your pc and when you printed everything it ran sluggish? well a wireless printer puts that burden on your network instead of your own pc.
As far as G devices go they transfer at a slower rate, while the N rated devices transfer at the full 150/300 rate.
You can test this by disconnecting/powering off the printer and surfing the web for a hour or so. See if you see a difference.
Unless you printing all the time, a wireless G Printer should have very limited effect on your network.