It depends on the wireless router. If it's only a wireless B router, then you'll only get the maximum wireless B offers, 11mbps. If it’s a wireless G router, it's possible the wireless mode is set to B or mixed mode (in order to support B and G clients). In either case, you end up w/ B performance.
Bottomline, if you have a wireless G router, and only have G clients, then make sure the wireless router is set to G-Only.