In a thread about benchmarks we found out that the same CPUs under Linux have a higher Whetstone and Drystone MIPS ratings in the BOINC Manager than those CPUs under Windows.
This should theoretically result in running more efficiently, BOINC-wise.
If your system is a bit older and you do not have an abundant supply of RAM, you can also try Xubuntu, a more lightweight variant, or Lubuntu, an even lighter one.