Your post shows that you use PowerMac, Mac OS X (10.7.3). However, PowerMacs cannot run any Mac OS newer than 10.5.x, so at least part of your posted information is inaccurate. Only iNtel Macs (NOT PowerPC Macs) can run Mac operating systems newer than 10.5. You should correct your "Products" in your Profile page to show the specific Mac you use.
Now for the solution to the problem about which you posted.
You can get your camera working again using the following:
• Use Apple's http://support.apple.com/kb/HT2090 troubleshooting.
Be sure to test more than one Apple app in more than one user account.
However, don't use the "Guest" account for testing. If your Mac has
only one account, create a new user account for testing.
• The proper SMC/PMU "reset" often restores normal function.
• Use your Mac's Disk Utility to repair permissions and then
download and apply the latest Combo Update, for your Mac OS,
the current one is at: http://support.apple.com/kb/DL1484
• Repair permissions again immediately following the system restart
that completes the Combo update.
Properly applying the Combo version of updates can sometimes correct software problems, but, if your trouble is caused by a hardware problem, there are no user serviceable parts of iSight. For hardware help, contact an Apple-Authorized Service Provider for service.
Mac Pro Quad Core (Early09) 2.93Ghz OSX 10.7.3 24" LED Cinema Display