Wirelss router. Simple, fast, and it works. Making a wireless network with a dongle and an Airport card would probably work, but it would most likely be real slow. A router is probably cheaper than a USB dongle too.
I have a Linksys router and a couple Linksys access points. I've had no real problems with mine. I'm sure others can recommend other brands that you can choose from.
Use this one to support 802.11G&B stuff :
http://www.compusa.com/products/product_info.asp?product_code=299891&csearch=&cmid=&pfp=srch1. A little more expensive, but will give you highspeed if you have an AirPort Extreme card.
Use this one for only 802.11B :
http://www.compusa.com/products/product_info.asp?product_code=282388&csearch=&cmid=&pfp=srch1. It is cheaper, and will work with pretty much any card on the market, except 802.11A ( which I haven't seen in a long time ).
The 802.11B only one is half the price of the one that supports 802.11G. If money is an issue, there is no choice here. Get the 802.11B model. If money is no object get the one that supports both 802.11B AND 802.11G.
All of the USB dongles are AT LEAST $30, so I would spend the extra dough and get a router, you will be much happier in the long run...
Hope this helps.