Well that's a matter of money and preference.
Option 1: Considering that your sig-rig already has a WD150, the best cheap option would be to put it in a raid array with another WD150. This array then will be used for your OS and programs. This will not be as fast as 2 300Gb raptors, but a lot cheaper.
For your other file's I would suggest a ordinary 7200rpm drive. The speed of these is fast enough for storage. I would suggest a WD5000AAKS or a HE103UJ.
Option 2: Also an option is to get, two 300Gb Raptors and put them in a Matrix Raid array.
200Gb raid 0 for your OS and Programs & 400GB normal/Raid0/JBOD for files.
The speed of your Raid0 space will be somewhat like this.
This options can provide some redundancy for your files, if a raid 1 is used for the file partition.
Option 3: The third and last options is to abandon the Raptors and get a SSD for your OS and programs. Along side this one you could get a 7200rpm drive for your files. This option is the most "future proof" considering that SSD are slowly but surely replacing hard drives. The only drawback is the price per GB and limited storage space. But the price's of SSD are getting lower by the day, so you could in a while get another one and put it in a Raid array for even faster speeds. I would suggest a OCZSSD2 or a X25-M, but this depends on your budget.
The X25-M
I hope that this helps to make a choice.
Greetings.