I have an iMac G4 and need to connect it to the internet. Do I need to buy an airport card or can I just use a normal USB wifi stick?

You can use a wifi stick.

Miriam

I am not sure if you still find the airport card for the G4. And probably it will be slower than a USB stick.

Check your Wifi Router to see which Frequencies and protocols (802.11 ... a b g n ???) and buy a Key compatible whith the standards you have.

Will this need to be a special mac compatible one or will they all be fine?

You need to see "works on Mac" on the box. All such deveices require a driver. The adaptor would work in many cases except for the lack of Mac drivers. Some companies have the proper Mac drivers on their web sites, some shuffle you off to suspect third-party sites, and some have no Mac drivers at all. If you want to buy one adaptor, not have to return it or send hours searcing the web for a driver that works, look at this one:

http://eshop.macsales.com/item/Newer%20Technology/MXP2802NU2C/

It is best for iMacs with USB 2.0 ports. If you have only USB 1.1 ports, you can save money with this one:

Newer Technology MAXPower 802.11g/b Wireless USB 2.0 Stick Adapter Share, Surf, & Stream Anywhere!

Allan is correct look at the box.

Miriam