The dual core i3/i5's with HT (hyperthreading) perform about equivalent to a 3 core processor.
fritz chess benchmark …
i5-750 … 8253
i5-661 … 6325 … (76.6% of the i5-750)
If one of the 4 cores of the i5-750 was disabled, you would end up with about 75% the performance of the original 750, or about the same as the 661.
In this other benchmark of
multitasking they ran the Crysis benchmark while a RAR process was running in the background loading up one logical processor.
i3-530 (2.93) … 42.5
G6950 (2.8) … 19.9
The G6950 is a dual core version of the i3 without HT (and a slight clock disadvantage). The G6950 saw its Crysis frame rate drop to 1/2 with the RAR task running at the same time.
The i3 was over 2x as fast as the G6950 while running RAR in the background, meaning that the i3 was still performing like a dual core processor. In fact, the i3 even while running RAR in the background was faster than the G6950 dual core with no RAR task running in the background. So, the i3 basically performed about like a 3-core processor in this test of multitasking.
There are different ways to skin a cat. Intel needed to provide a jump over their previous dual cores, so rather than a 3 core they went with HT and high clock rates on the i3/i5 dual cores.