Press C during startup Start up from a bootable CD, DVD, or USB thumb drive (such as OS X install media).
The only option for retail media is OSX 10.6, there was no 10.6.3 retail media, since all incremental updates are free. The sensor you threw out is a thermal sensor and will make the iMac fans run at full tilt forever, even if you do get it booted. You should be able to hold C down and get it to boot. The "Folder to Question mark" icon flashing is because it cannot recognize a disc, which is probably the hard drive. It is possible the drive is not functioning, but if it recognizes a command and spools, it is unlikely.
The hard drive should be clean, but if it was formatted any way but OSX Journaled, it will not be able to write to it. You would have to start the install and use disk utility to format the drive.
Try booting without holding anything down, and see what happens. It is a long shot, but if no OS is found, and the computer starts searching for other media, it'll find the OS in the DVD drive. The flashing issue is normally when the start up disk is missing. Worst case, find an apple store and ask some questions, or send me your drive, and I'll install Snow Leopard, and update to 10.6.8 and ship it back.
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