iMac Help PLEASE. Wireless signal keeps dropping.

My iMac G5 is very finnicky when connecting to my wireless router.

Sometimes, it won't pick up ANY wireless networks at all, and I have to reboot the machine before it will do so.

Other times, it will drop the signal, etc, for no reason.

Now, I have a MacBook as well, and that holds and maintains the connection with no problems.

NOTE***: Now Windows tells me its connected to the card, but I can't connect to the internet at all.(tried through boot camp)

HELP!
I decided to reformat my machine the other day on the offchance that something just got installed improperly, but no joy.

Any suggestions as to what may be causing this?
It's driving me crazy!

Thanks!

Update: Some friends and I have surmised that it must be an issue with the wireless card.

It's not under warranty.

How expensive is an Airport card? Also, which would I need, and how difficult is it to replace the card itself?

thanks!

Some things to try:

1. Change wireless security to WPA2 if available.
2. Enable SSID broadcasting if it's disabled.
3. Use a program like AirPort Radar to determine if one of your neighbors is using the same channel as you. If so, change the channel.
4. Put the router in a different location. The higher up the better, and as central as you can in your home.
5. Make sure the router is not near any cordless phone base station and away from microwave ovens.
6. Check your router's firmware version and make sure it's up to date.
7. Run Software Updates and ensure that you've downloaded the latest patches.

My router's firmware is up to date, my software on the Mac is up to date, I am running WPA2 ( of course ).. and still nothing.

I know its a hardware issue because there are about 5 other machines in the house connected wirelessly, and this machine is the only one with problems.