What kind of wireless adapter is best for gaming?

I know that a direct wired connection is optimal for gaming, however that is not an option for me. Since I must go with the wireless option, I was wondering if I should choose a PCI adapter or USB? Also, there are apparently different types such as B, G, and N -- which of these is better for gaming? My wireless router is a Linksys E1000 which I believe is a "100 mbps" router. Unfortunately, the router will be located downstairs, almost directly under me. I'd like an adapter that can reliably transmit through walls, and I am willing to purchase some kind of signal booster if necessary (so don't base your recommendation on how far the adapter works). I would appreciate any information and/or specific recommendations as I am looking for an adapter that can handle online games such as World of Warcraft.