+1 don't use two different drive sizes on raid 0... it is meant to split data equally between two identically sized drives for performance reasons. Obviously once you exceed approx 600gb in data your writing soley to the WD which is no longer raid 0 (hence why it's not possible).
As the 2 replies before me say, you can't do what you are looking to do with what you have...
That said, the 596GB you are seeing is probably not the size of the array created 9as that number makes no sense for the hardware you listed) but rather is probably the size of UNALLOCATED space still available (you made a RAID array with 320GB of the WD + the 320GB Hitachi leaving some number less than 680GB on the WD unused).
With 2 drives of unequal size your choices are to leave them separate or if you need a single large volume, JBOD.
You can build an array with those drives but even I would question doing this.
RAID 0 reserves space on all drives equal to the size of the smallest drive. The rest of the drive is not useable/lost.
Drive manufacturers use 1,000,000,000 as their gigabyte - 320 Gbytes is 320,000,000,000 bytes. Computer people use 1,073,741,824 bytes for a gigabyte.
320,000,000,000 divided by 1073741824 which is almost that 596Gbytes you see (the lost amount is due to the rounding up the manufacturers do also).
You are seeing the space RAID 0 created using 2 very different drives. If you want that space back either use 2 1TB drives or break apart that RAID 0 array and go back to native drives.