I need a wireless internet adapter

[o_O]

Now however, in my experience USB adapters are significantly less powerful than PCI adapters. They give a much slower connection speed (maximum is typically 54Mb/s) so while they would suffice for an Internet connection, playing games on a LAN or sharing large files and folders could suffer from it.

All wireless goes at 11Mbps-54Mbps. It's only the new 802.11n that will be able to go at 100Mbps, and that's still new technology so I don't trust it, not to mention it's extremely expensive!

While USB2.0 is capable of the 802.11n standard at 108Mbps, having used both PCI and USB, I've found that it is less stable and slower in practice than the PCI adapter. The reason is that USB draws power from the USB port only, which means that the more USB devices you use off of the same controller, the more you'll have power throughput issues. The PCI adapter, on the other hand, draws from the PSU, so there are no power problems there.
The n standard itself is actually perfectly reliable though, the cause of wireless problems is generally poor hardware, which brings me to my next point.

With wireless, you get what you pay for.
Brands to avoid in the wireless department are, unfortunately the cheapest, which include Netgear and DLink. Both of which I used for some time. Really the only brand I fully trust now is Linksys.
In New Zealand, most wireless adapters come in USB and PCI variety, with the PCI being around $NZ10 (about $US6.90) cheaper.
For the PCI adapter I paid $NZ70 ($US48), so you shouldn't have any trouble finding one within your price range.