The main difference is the manufacturing technology. CPUs of the series T5xxx and T7xxx are of the Merom core and manufactured on a 65nm structure. The T8xxx and T9xxx are of the Penryn core and manufactured on a 45nm structure. Because of the reduced structure it was possible to increase the L2 cache by 1MB (on the T8xxx series) resp. 2MB (on the T9xxx series). Difference between T5450 and T8100 T5450: 1.66Ghz, 2MB L2 cache, 667Mhz FSB, Merom 65nm T8100: 2.1Ghz, 3MB L2 cache, 800Mhz FSB, Penryn 45nm these 2 processors are interchangeable because both uses Socket P (Santa Rosa platform).
Decided which is the best for urself T8100: 35W | 105*C | 1.050V-1.23V P8400: 25W | 105*C | 1.00V-1.250V In my review P8400 consumes less power, less heat stands on centrino2 so it GREAT !!
The T8100 is based on Intel's newer processor cores, dubbed "Penryn". They feature a smaller 45nm manufacturing process, as opposed to the "Merom" core on the T5xxx/T7xxx series. They also include a new instruction set, SSE-4, but I doubt any software is making much use of that yet. Most notably they do consume less power than their predecessors. So the "reason" why the T7500 is 2.2ghz and T8100 is only 2.1ghz, is that they are from two different processor families, and apparently Intel deems that the advances they put into the new Penryn core, allows them to have the cheapest member of the line running at a lower clockspeed than the T7500. Be it performance or power-saving that is the reason behind it, only Intel (or benchmarks) can tell.
Technically, they are nearly the same. However, if you want to see the differences, I suggest you look at these sites. http://processorfinder.intel.com/details.aspx?sSpec=SLA44 http://processorfinder.intel.com/details.aspx?sSpec=SLAP9