I’ve been testing the 2010 iMac 27" with the 2.93GHz Intel Core i7 processor option. It delivers performance similar to that of a quad-core Mac Pro.
A full report and comparison of the 27" iMac is planned, but see my discussion of why a Mac Pro is still a better option in most cases.
See also my various tweets on the 2010 27" iMac quad-core.
Apple solid state drive (SSD) option
See my previous notes on the iMac SSD option— the Apple SSD is not a dog, but it’s not very fast as solid state drives go, and if you want to swap it as part of an upgrade, it’s labor intensive and will cost a lot more, due to the way Apple installs it. Don’t go there.
You do not need to order the Apple SSD option to have an SSD in your iMac. Any 2010 iMac 27" model can take a solid state drive.
Processor grunt — CPU cores and clock speed
For more on this topic, see my post over at B&H Photo: Apple 2010 iMac: Which Model for Photography?.
The dual-core 3.6GHz model can be a good choice: it will run some tasks faster than the quad-core models. But as soon as those two CPU cores are "pegged," that's it.
Think of the 3.6Ghz dual-core model as being roughly equivalent to a 3-core Intel Core i5 2.8GHz model, once clock speed is taken into account, but performing better on tasks that are not efficient, like saving or opening files in Photoshop. Thus, the choice is not cut and dried