When you set up a RAID 0 configuration you interleve the data on two drives. Your maximum RAID 0 drive space is double the smaller of the two drives. Ideally you should use the same model of HDD from a single OEM. RAIDing HDDs from two different OEMs may prove troublesome if not impossible if the drive speed isn't matched.
RAIDing two HDDs from a single OEM of 750 GB size should produce a "single RAID drive" of approximately 1.5 TB.