You will see a decrease in performance. The Turion 64 MT-40 is 2.2GHz. The 3700+ is 2.4GHz. But the Turion 64 will consume less power, if you can get PowerNow! to work properly. AMD laptop CPUs uses PowerNow! instead of Cool 'N Quiet.
MSI RS482M-IL: The only board we've found supports Cool'n'Quiet on Turion 64.