The short answer is no. Although both of the processors you are comparing are Merom-based, the each use a different socket type - thus they are not compatible. A socket P processor will not boot in a socket M based laptop.
Intel® Core™2 Duo Processor T5500 (2M Cache, 1.66 GHz, 667 MHz FSB) uFCPGA, Socket M, Tray
Intel® Core™2 Duo Processor T5250 (2M Cache, 1.50 GHz, 667 MHz FSB) TJ, uFCPGA Pb-Free SLI, Socket P, Tray
In my opinion, ark.intel.com is a pretty good source for comparing processors and chipsets. You might also check with Dell to see which processors they suggest as an upgrade path, as well as to check to see if a BIOS update would be needed when upgrading the processor in your particular laptop.
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