There was 3 differences between the Intel® Core™ i5-660 and Intel Core i5-661.
The biggest difference between them was that the Intel Core i5-661 had a faster clock speed on it IGP (Intergraded Graphics on Processor) at 900MHz vs 733Mhz on the Intel Core i5-660.
The Intel Core i5-660 has support for TXT (Trusted Execution Technology) and VT-d (virtualization with directed I/O) and the Intel Core i5-661 doesnt support these technologies.
You can see the information on these processors here http://ark.intel.com/compare/43553,43550
Intel Enthusiast Team