All I can say about cross-compiling is that I tried to compile sparc64-binaries on a amd64-PC and that resulted in a huge mess. I think I will give it a shot again sometimes in the future but for the time being I don't feel like telling other people to do the same.
I installed Gentoo a few weeks back on a netbook having an Atom 270 and it worked fine - it just lasted 4 times longer than usual, but it was still acceptable.
What you could do instead of cross-compiling is to use "distcc" when you emerge something. "distcc" is fully transparent to "emerge" and it spreads out the compilation as well to other PCs connected to your network.
I used it on some PCs and it worked fine and it is for sure easier to use than cross-compiling, especially once you have a working environment.
The guide for distcc for cross-compiling: http://www.gentoo.org/doc/en/cross-compiling-distcc.xml
Hope it helps!