You will find that Gigabit cards in the groupwise servers is the way to go
anyway.  If you figure out the SAN client software costs it will offset most
of the hardware cost for the Gigabit.  I have seen numbers has high as
55MB/sec on this even on a PC100 Intel Machine if you have good switching
technology.  You should be able to restore these in 30 minutes with good
hardware.  1GB per minute is easily achieveable.

You will find that the SAN connected disk can probably push the data over
the gigabit to the tape drives fast enough to keep them going.  Also, if the
Intel Machine is at least 1GB Memory and 2 600Mhz processors it will suck
the life out of those groupwise servers.  Depending on how many you have and
your overall TSM direction, the RS/6000 may not be required.  But, in any
case the FC 3590 is a good choice, expensive, but a good choice.

30GB is not a reason to implement SAN TAPE 100GB+ is.