You'd be wise to put the OS and the software on one drive, and have the 2nd drive be the 'data dump.' I would also consider how the drives will be interfaced to the controller. SATA? ATA133?
Back in the days of the PII450 I designed a video editing box for a local cameraman, it involved a SCSI UW Raid5. Back then, trying to capture 640x480 streams was VERY combersome.
BTW, will she be using a capture card or FireWire? I know there are some mean analog capture cards out there that have some great DSP's and a good amount of buffering ram.
Honestly, try to find out
what she wants to capture, first. Then you can more accuratly determine the kind of machine that you need to build.
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At home, my g/f does a lot of low-quality capture from a Hi8 videocam to 320x240 and 640x480 and reworks them with premier and combustion. She is still using a PII450 with 384MB of ram, and (1) 40GB drive. Yeah, things can be slow but it still works!
-SF