It might be a good idea to investigate why your wifi throughput is so poor. If there is interference or poor signal quality with 11g, there's quite possibly going to be problems with 11n also.
I have a Linksys 54g WAP/router and I get a consistent real-world 20-25 Mbps out of a theoretical 54Mbps with two PCs, which is quite acceptable for a domestic situation (I've seen 35mpbs, but only when I've stopped all but one device). The signal is passing through a couple of walls or floors, but still strong. I've got a small extension aerial for each desktop, so that the aerial isn't shielded by the PC case.
I can't do that for the laptop card, so it's no surprise that it has poorer reception. I haven't tested the x-box throughput, but the kids aren't complaining, so I guess it must be OK!
SD