Unless you are an expert in building cross compilers / tool chains :
It's not recommended to build one yourself.
And a standalone "gcc-mips" won't do.
The minimum for a tool chain is :
binutils, "gcc-mips", a libc ( Uclibc glibc eglibc etc.),
and headers from the "preferred kernel".
The recommended tool chain is : crosstool-NG http://crosstool-ng.org/
"crosstool-NG" is the only tool that's updated regularly.
Last update is : 20 Nov 2011.
GCC version : Probably a version that can be used for your kernel.
General purpose : gcc-4.4.4 ( A later version like 4.6 : New software only.)