So my E6750 is running at 3.2ghz (400x4) with 1.30v (shows in CPU-Z, in my bios it's 1.3250)

If I lowered it to 2.66ghz(stock) or 3.0ghz I can run both stable at 1.22V (shows in CPU-Z)

Is that normal? I mean that's a big voltage difference between the two speeds and the speeds difference is not that big. Also, between 2.66 and 3.0ghz (both at 1.22v) which one will produce more heat? or they should be about the same since they're both 1.22v?

A drop in EITHER speed OR voltage will drop temps.

My Q6600 B3 for example:
2.7 @ 1.15v
or
3.0 @1.25v

Thats also quite a big jump in V for 300mhz more
Most chips will have a sweet spot for volts vs Ghz, its just a matter of trial and error.

It'll be hotter at 3.0ghz 1.22v then at 2.66ghz 1.22v. Reason being is the Processor draws more current at a higher frequency, thus producing more heat.

Frequency makes minimal difference in power consumption if voltage remains the same. A change in voltage makes a large difference in power consumption, and therefore heat output. But to answer your question, at 3GHz it will run hotter, but likely no more than 1-2C hotter at the most.

Here's a simple formula for calculating approximate power consumption based on TDP:

Power Consumption = TDP * (current speed / stock speed) * (current voltage / stock voltage)^2