I need to know what is the difference between? & which is faster?
of these 2 processors:
Intel Centrino Core2 Duo 1.8 GHz T7100
Intel Centrino Core Duo 2GHz T2450
and generally between Core Duo & Core2 Duo processors…
thnax in advance 
Do not even have second thoughts my friend. Go with Core 2 Duo.
Core Duo 2 offers 3 different advantages over regular Core duo.
For one Core duo is 32 bit and Core duo 2 is 64 bit processor. Also, Core 2 duo CPU’s are cca. 20% faster per clock. They also have more cache 4MB’s vs 2MB’s (note that some older Core 2 Duo have 2 Mb’s only). The notebook Core 2 Duo’s are supposed to be a lot more energy efficient (which translates to less heat generated).
Here is quote from Pinoy Reviews:
Dual core is just the term used for processors with two physical CPUs. The early Intel Pentium 4 processors only have one core so when it says, Pentium 4 2.4 GHz, the speed is only 2.4 GHz while when you say Dual core 2.4 GHz, it means you have two processors running at 2.4 GHz each so your computer can run faster. I used to think that Dual Core is the proprietary name for Intel's new processors but AMD also sells dual core processors so Intel's marketing name for their new dual core processors is Core Duo and Core 2 Duo is the second generation of those processors.When you look at it in a manufacturing point of view, there’s really not much difference between Core Duo and Core 2 Duo processors since both use the same production process and some motherboards are compatible with both Core Duo and Core 2 Duo, but what gives Core 2 Duo processors the edge is the architectural changes on the design of the chip that can give it more processing abilities.
I have had the chance to upgrade my custom built desktop PC from the previous Core Duo processor to a Core 2 Duo processor and I can say that performance wise, the Core 2 Duo processor can perform faster and multi-task better. Aside from the faster processing speed of the Core 2 Duo Processor, it’s also prone to less heat and less noise than my previous Core Duo processor. I have monitored the internal state of my desktop PC when I was using a Core Duo processor and I have to tell you that the Core Duo processors runs very hot, I thought I bought a defective processor but my Core Duo laptop also runs very hot as when compared with my upgraded desktop PC running Core 2 Duo and my other laptop which has a Core 2 Duo processor.
In terms of price, laptops and desktop PCs running Core Duo Processors are usually a lot cheaper compared with those running Core 2 Duo processors. Let’s just say that if you’re going to run a lot of Video Editing and you’re going to do a variety of tasks on your PC/laptop, Core 2 Duo Processors is the best choice but if you’re just looking for something to browse the net and do simple word processing, Core Duo Processors will do it just fine.
Also the Core 2 Proccessors have additional SSE improvements over the Core Duo which will help in applications that are Optimized to take advantage of them (Like QuickTime and iTunes encoding) The Core Duo is still a very fast Processor ( I have a 1.66Ghz Core Duo in my Mac Mini) but unless you find a KILLER deal on the Core Duo system get the Core 2.