Add a Hard drive

I have an XPS 600 (bout 3 years old) It currently has two Hard drives (150 gig each) which are configured as raid 0 I beleive as I only show one 300 gig hard drive.  I want to add a 500 gig HD that I have just to use as additional storage (actually have two but only one open slot) space as my HD is almost full.  Installing the HD is no issue, thats done however configuring for use is where I'm not sure what to do.  The PC currently does not see it.  I went into the BIOS and turned it to on and still nothing.  The drive I'm adding is a Seagate Barracuda 7200.10. Didn't want to set to raid for fear of losing the data I currently have.  Any assistance or advice would be great.

Thanks,

Mike