Hi-
That card is not intended to be used for single drives.
It is a RAID controller, and as such, will most likely require two or more drives to be connected.
Even with a second drive, I cannot say as if it will be trouble free.
The Silicon Image chipset is really primarily for the WinPC market, though they are sold as Mac compatible.
Flashing BIOS is not something that should be necessary in a Mac. Updating the ROM may be done, but shouldn't be necessary.
I have seen various reports of difficulties with that chipset. The chipset is listed as 10.3 and 10.4 compatible, so it may be that it is not 10.5 compatible......... might need some drivers.
Might have a look at this discussion:
http://discussions.apple.com/thread.jspa?threadID=1985879&tstart=15
Other information on possible drivers that may help are here:
http://www.xlr8yourmac.com/feedback/MacPCI_eSATAcards.html