ok go to the advanced video properties, go to the tab for Nvidia, then go to the section titled Desktop Utilities.
Check the box saying 'Enable Desktop Manager'. Then hit the Configuration button, then under the 'Effects' tab you will see a checkbox for 'Enable taskbar transparency'. Check it, set the percent, hit apply and youre done. You may have to restart for the nView service to kick in.
Ya gotta tweak the right percentage, i think 40% looks pretty good, while still being able to SEE whats on the taskbar!
By the way, you MAY need newer drivers to have these options. Having a GF3, im sure you keep fairly up to date, yes? I believe all the detonator versions have it. If you cant find these options, just get the latest drivers from Nvidia (be sure to uninstall first!)
best of luck, have fun. lemme know if you run into trouble