I'd recommend looking for an old AirPort card on eBay. Make sure it's a regular AirPort card, and not one of the newer AirPort Extreme cards. This will give you 802.11B access, which is theoretically rated at 11Mbps but in reality is slightly lower. This should easily suffice for most internet connections, but large file transfers will be a little slow (think USB 1.1 speeds). Third party wireless cards are notorious for causing crashes and kernel panics, which is why the AirPort card is usually probably the better option, although since they're no longer produced they can be a little pricey on eBay. The speed of the computer shouldn't come into play when it comes to this particular network connection, however you may find browsing Flash-intensive sites a little slower than a modern computer. This wouldn't be any different if the machine was hardwired to a modem though.