I7 3667U vs Core2Duo T9800

the i7 wud be slightly faster, as it has a superior architecture and uses HT....but if u'd want a rough estimate....shud be around 10% faster   [no proof...pure shawnproximation   ]

anyway...the biggest deal with the i7 is that it consumes just 17W, half that of the T9800  

At the same speed you can expect that the 3rd generation Intel® Core™ processor would out perform an Intel Core 2 Duo processor by more than 30% and more if you are going the IGP (Intergrated Graphics on Processors). Now since one of the ways that the Intel Core i7-3667u matches the speed with the Intel Core 2 Duo T9800 is due to Turbo boost which may not be up to its full speed at all times due to the number of cores and temperatures. In the end I would expect that you would see performance that is at worst comparible most of the time and much better battery life from the Intel Core i7-3667U. http://ark.intel.com/compare/64898,37005