surprised that I'm the only one here using xACT to do this on my mac.
xACT uses LAME, and I usually set it to the highest VBR setting...which ends up being around 256k most of the time.
So far, I've dropped about 750 gigs of FLAC & SHN files onto xACT (usually about 30 or 40gigs at a time), and it's produced every file without a hitch...
Highly recommended.