First of all, I don't know if BS2, BS3 and BS4 accept SDHC, as this is a new format.
All of the systems above accept SD cards up to 4GB without any problem. With larger memory, the problem is just, that the access to the files will be quite slow. IMO, a 2GB SD card is more than enough for these beosounds. I use 1 GB cards for my BS3, and I never stock it up to the maximum, as it gets difficult to search for files/songs. I have one SD card for classical music, one for jazz and one for random music. IMO, 2 GB of music, even at 320GB is more than sufficient.
Music washes away from the soul the dust of everyday life. - Berthold Auerbach