Keep in mind that the compatibility between the Intel processors and any laptop system is determined by the laptop manufacturer, so in this particular case, you need to contact DELL* so they can provide you a list of fully supported processors for your laptop system.
T2300 is a socket M processor, and T6600 is for socket P. So, T6600 won't work in your laptop. The fastest socket M processor is T7600, and that may or may not work depending on the chipset and BIOS in your notebook. Core Duo T2600 and T2700 are very likely to work. One more thing. T2300 came in two different versions - BGA (soldered) and PGA (socketed). Make sure you have PGA version before upgrading.