Can I replace this processor with a dual core ? What is the best for unit replacement.

Well, it depends on what socket the original Mobile Sempron is. The socket 754 Mobile Semprons are only upgradeable to single-core Turion 64s, whereas socket S1 Semprons are upgradeable to Turion 64 X2 dual-cores.

A quick-and-dirty way to tell is to look at the RAM in the computer. All socket 754 computers will use DDR RAM while socket S1 ones use DDR2 memory. If the modules are unmarked or your OEM's website doesn't tell you the type, there are tools like CPU-Z that will tell you what your chip's socket is.

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Cpuz is here

http://www.cpuid.com/cpuz.php

the Download button is on the top left part of the page and should say
Download Latest version and under it CPU-Z 1.39 or higher incase they update after i post this.