"Would it be possible to change it for an intel core 2 du0 2GHZ T7300?"
In a word - yes.
That said, you're still looking at a 10 year old computer. I too own a 2006 iMac. About a year ago, the display started tiling - subtle at first, then more pronounced. My research dug up a design flaw where the solder holding the graphics chip to the motherboard disintegrates over time causing overheating. It's a fairly easy fix if you're comfortable taking one of these apart. However, once fixed, it's still a 10 year old computer. I purchased a late-2012, maxed out the CPU, GPU, and RAM. It runs Sierra, and every task I can throw at it, effortlessly.
If you have the means, I would give serious consideration to purchasing one of the newer iMacs.