I"m assuming your referring to the Intel Pentium E6600 (http://ark.intel.com/products/42807/Intel-Pentium-Processor-E6600-2M-Cache-3_06-GHz-1066-FSB) & the Intel Core 2 Duo E6600 (http://ark.intel.com/products/27250/Intel-Core2-Duo-Processor-E6600-4M-Cache-2_40-GHz-1066-MHz-FSB)
I believe the Core 2 Duo system will be a little faster due to its larger L2 Cache, even though its on a larger lithography.
It really comes down the specific applications you're running.
If its just Outlook and Excel, then either one will be fine