I did some research on this:
From the motherboard socket LGA1366 there's a large number of CPU's supported:
- Core i7-960 or 970
- Core i7-990X
- Xeon W3580, W3690 or W5590
- Xeon X5690, X5687 or X5698
The top ones from this list are the i7-970, X5690 or X5698
The i7-970 and X5690 do have 6 cores and the X5698 only 4 cores.
The X5687 does have:
- Higher Max Operating Temp (80.4C vs 78.5C)
- Higher Turbo clock speed (3.86GHz vs 3.73GHz)
- Slightly higher clock speed (3.6GHz vs 3.46GHz)
- Slightly better overclock speed (4.61GHz vs 4.47GHz on Air, and 3.6GHz vs 3.47GHz on Water)
If your looking for clock speed, X5687 wins
If your looking for cores, the X5690 wins
- Your computer is a iMac so I doubt upgrading your hardware is possible, but if it is you can try (Though the OS might conflict with it, as it's Mac OS and we all know how limited Apple is when it comes to hardware.)
- You need to make sure the RAM is compatible with the new CPU
- Also look carefully at the Power Usage and Max Temperatures of the new CPU!
As for installing a CPU, there's lots of guides out there (I believe here on iFixIt too) on how to upgrade CPU, as for me I didn't experience problems when upgrading from i5-3470S to i5-3570.