To provide all the options, more information is needed.
Did you check if your machine already has wifi?
You may want to post machine info.
blue apple > about this mac > more info button. Click on thehardware line. It has a little triangle in front of the work hardware.
What encryption are you using on your existing router?
Your best bet in the Ethernet Bridge. All you need is an ethernet port. The usb dongle will require an OS upgrade.
Ethernet Bridge
Ethernet Bridge "The versatile Wireless-G Ethernet Bridge can make any wired Ethernet-equipped device a part of your wireless network." from linksys example: http://www.amazon.com/Linksys-RE1000-Wireless-N-Range-Extender/dp/B005FDXMJS/ref =sr_1_1?ie=UTF8&qid=1314563758&sr=8-1
Airport Express Airport Express is an external unit and you can configure it as a bridge. 802.11n ( I bevieve you will need a more modern version of Mac OS X to configure. Sadly, Apple uses a configuration program to configure, not a web browser. )
2. USB dongle
Introducting RokIt, a new wireless USB adapter that is compatible with Mac OS 10.3, 10.4, and 10.5 Leopard. http://rokland.com/store/product_info.php?products_id=319
Newer Technology MAXPower 802.11g/b Wireless USB 2.0 Stick Adapter $15.99
http://eshop.macsales.com/item/Newer%20Technology/MXP2802GU2/
Newer Technology MAXPower 802.11n/g/b USB Adapter. The easiest way to add Wireless Connectivity to ANY computer! $29.99 http://eshop.macsales.com/item/Newer%20Technology/MXP802NU2C/
3. I'm not sure which airport card you need.
It's likely to be:
Apple Airport Card + AdapterApple AirPort Carrier Card Adapter Pulled p/n M8753G/A or 076-0804 To use an AirPort Card with certain iMac models (G3 iMacs and others), you will require the use of the AirPort Card Adapter (Airport Card sold separately). http://www.swnetworksolutions.com/m8753g-a.html
http://www.info.apple.com/images/kbase/107908/107908_11.jpg
Technical Details Wireless!! This Airport Card supports a wireless data transfer rate of up to 11Mbps with enhanced 128-bit security and firewall protection support! The Apple AirPort Card features cutting-edge wireless networking technology that gets you on the Internet from practically anywhere without cables so you can stay wireless! Its 2.4 GHz spread spectrum radio output power complies with IEEE 802.11b industry standard to allow for interoperability with other 802.11 compliant products!
* Original Apple-Mac Airport 802.11b Wireless Card General Features: Up to 11Mb per second
* 2.4GHz spread spectrum radio output power Enhanced 128-bit security and firewall protection support
* Works with the following: iMac (Slot Loading) (requires adapter) * iMac (Summer 2000) (requires adapter) iMac (Early 2001) (requires adapter)
* iMac (Summer 2001) (requires adapter)
* Support only the older WEP standard source of info
[ WEP is not recommended for secure communications. Robert ]
http://support.apple.com/kb/HT1621
You can look around for wireless networks. See: iStumbler - 98 find AirPort networks, Bluetooth devices, Bonjour services http://www.versiontracker.com/dyn/moreinfo/macosx/17572 Robert