The key is the backplane, not the drives. You should be able to get u320 speeds if you have enough u160 drives.
Does five drives count?
I was just hoping a slightly newer card might be available that'd give me a speed boost for cheap. :-)
Again, like John posted, it depends on whether your backplane(s) can feed the u320 card and whether its bus can take the bandwidth.
AFAIK, the back plane is rated at U320 speeds. The unit is a Supermicro CSE-M35S (SCA) which is listed as a U320 rated unit.