MAC address filtering is not at all secure and extremely easy to break into.
Alternatively you could probably set up a second wireless network in your router config using a lower encryption setting like wpa and only whitelist the address of your imac g3 so only it would be able to connect.
If security isn't a priority, then why even use MAC address filtering? Why not just use a WEP key or no encryption at all?
Alternatively you could probably set up a second wireless network in your router config using a lower encryption setting like wpa and only whitelist the address of your imac g3 so only it would be able to connect.
I've had good luck with this Tenda WU311 Wireless Stick on a G3 iMac:
http://www.amazon.com/Tenda-W311U-Network-adapter-Hi-Speed/dp/B001BBURAA
Original drivers are here:
http://www.tenda.cn/tendacn/DownLoads/show.aspx?downid=764
Updated drivers are here:
http://download.wireless-driver.com...2/RTUSBD2870-3.0.3.0UI-3.0.0.0_2010_10_29.dmg
There's a newer driver out there for Leopard; you need to find the Ralink Utility for the RT2870 chipset.
The best thing to do, speed-wise, is to get an Airport Express, join it to your wifi network, then connect it to the iMac through ethernet.
That way, you don't have to take your Mac apart to put a new wireless card in, and you're not limited by the 12Mbps USB1 ports. And you can still use modern encryption standards like WPA2 on your network.
Or, you can get an old Airport, connect it to your router, and make a new WiFi network for your old Macs.