The 128 GB limitation applies to the onboard IDE controller, and not to an ATA-133 controller card that supports 48-bit logical block addressing, which enables recognition of large drives. If connected to an ATA-133 controller card, partitioning the 320 GB drive is your choice, although I prefer to partition larger drives for the ease/speed of routine disk maintenance. The Acard AEC-6280M ATA-133 controller card for Macs is consistently priced lowest
here.
Sonnet and SIIG also market Mac controller cards, but whichever brand that you choose to purchase, it must be the Mac version, because the PC versions of these cards aren't compatible. Bootable controller cards must be firmware-based and not driver-based. The card's ROM is read at boot, so that connected drive(s) are functional. If the controller card required a driver to be read from the hard drive, the sequence for doing so couldn't occur, because the hard drive isn't recognized until the controller card (to which it's connected) is recognized.