I am thinking of upgrading the current Core 2 Duo 3.33GHz to Quadcore

"Samuel - thanks for clarifying your question. my understanding is that the 3.33GHz processor in the 21.5"" iMac is an Intel Core 2 Duo E8600 CPU chip, using an LGA775 socket

there are Core 2 Quad processors using the same socket so I guess in theory your upgrade might be at least physically possible? I've not seen any reports of people doing this yet, but it sounds like an interesting possibility. I would worry on the other hand that it might require firmware support on the motherboard?? sorry don't have the definitive answer, maybe someone else can comment"

One problem with this is the 21.5" iMac has a 1066MHz system bus, and the better C2D processors are 1333MHz bus speeds. If you stuck a Core 2 Quad chip in the iMac, it would probably revert down to the 1066MHz bus speed, which would significantly reduce the overall clock speed of the new processor. But, if it did work at least you would have quad cores.

Well... since you're obviously willing to scrap the warranty anyway, I'd try to overclock the 21.5's E8600 (the 3.33 core 2 processor) first before I'd try to swap the chip (which could be both a heating and a compatibility issue). That particular Core 2 is the badest Core 2 Dual ever made. PC users have overclocked it to 4.00 ghz, 5 ghz and beyond. The iMac is a tight unit, but you might be able to safely overclock it through software by a few percentage points and see how it goes. If you are mechanically inclined, you might even find a way to improve the cooling system in the iMac.

"I have a iMac 21,5 to and I'm also interested upgrading my E7600 to a Q9550s if this is possible.

Since the Q9550s also uses 65W just like the E7600/E8600 and the bus speed is the same as the E8600 (1333MHz) which is a upgrade I think it might be possible. The only question is: ""Will the firmware detect the Quad-core properly?""

I will certainly wait until my warranty is expired, even if nobody have tried it, I certainly will. Of course I love to see anyone doing this, then I really know if it's compatible or not so I will have it clear for myself."

"That is something I doubted about to, but the 3,33GHz E8600 model is a 1333MHz, just like Q9550s. I think the firmware of the iMac will change the clock ratio for the processor to let it run at original speed.

E7600 = 1066Mhz > 3.06Ghz

E8600 = 1333Mhz > 3.33Ghz

Let's say the Q9550s is in my opinion the most equal to the E8600 in specs, If this would work it will be a nice upgrade!"

If the iMac can do a 1333 FSB, then I would think a Core 2 Quad would actually work. Assuming it is similar architecture as the Core 2 Duo being replaced. I read about someone trying this swap elsewhere, and they had no luck (no boot), but they used one generation older Core 2 Quad, which would have a higher thermal loading, and the chance of it being compatible was not that high in the first place.