I am certain Grub has no bother with a hard disk of 320Gb size. I routinely force Grub to boot from the end of my 500Gb and 1TB hard disks.

I have just wrote this thread on the maximum number of hard disk partitions with the new 2.6.28 kernel. I used a 1.5TB hard disk and generated 57 partitions. I have installed two Linux in the sdb56 and sdb55 partitions and boot them up successfully. They are at 1.1Tb position of a 1.5TB disk.

The 137Gb limitation is usually specific to the operating system and not a matter for the boot loader. If Grub is able to list the partitions it can boot them. This because Grub needs to search the disk to find the location of the last logical partition.

Having said the above it is possible if you are working with an old PC with a Bios limited to 137Gb (has to be very old though) then you may have a booting problem because the Bios would tell Grub there in no hard disk address beyond the 137Gb. However this limitation should disappear if your Bios allows you to use the hard disk in the LBA mode (Large block addressing). Can you check your Bios to see if the hard disk addressing mode can be switched?