The package sizes sent back are in the 1.5MB area so the upload speed for each server is not critical, we have several fast build servers hooked up to ADSL and they still provide a nice service.
And to answer jdgordon: we do separate builds on what compilers each server has: sdl, arm, m68k and sh, not by what bandwidth each server has. So while we _could_ enhance the build script to take that into accout as well, I'd rather not and just require build servers to have "fast enough" bandwidth.
We can always add servers for a while to test them and see if they are fast enough and simply remove them again if they end up slowing down the total instead of helping out.