When you RAID 0 two drives, only the smallest will be taken as a reference for the striped array. So in your case with the two Velociraptors you have, only 150GB of the 300GB drive will be used. It may be better buying two WB Black instead of the extra VR 300 GB. Keep in mind, RAID 0 is dangerous, as if one drive fails you will lose all data on the array. You will also need to reinstall Windows on the array, as when you create the RAID array, all data will be lost.
If its speed you’re looking for, and don’t need huge capacity, a Single Gen2 SSD will be much faster when compared to any HDD RAID array, you could get an OCZ Agility 120GB SSD, for the same price as that WD VR 300GB.