Were you planning on using the 15G drive as a OS drive?
DV is not dificult to capture - 5400RPM drives do it very well. For a software-only system like Premiere, most of your rendering will be so slow that again, drive performance will not be an issue. As to memory, DV is so highly compressed and the frames so small that it is very possible that there wil be no difference between 512Mb & 1GB or RAM - not 100% sure on that as it has been years since I've even fired that software up.
The real problem you are going to have is with pure real estate. It depends on how good an editor this guy is (many folks today - bizarre as it seems, "editing" is even "taught" this way by folks who should either know better or be fired - just capture everything and then "edit" with the software) and what sort of source material and final product he is working toward, but even 120G can get cramped pretty fast.
As a comparison: I just completed two projects, one with 6 hours of raw footage across 10 cameras (=60 hours of footage) edited down to a final project run time of about 3 hours single-screen, and another with 20 hours of two camera footage (=40 hours of raw footage, though it was actually 60 because we recorded a live switch, too) edited to 20 hours. Both were live at the same time, so if I were a lousy editor and just dumped it all to HDD I would have needed 120 hours of on-line time just for capture, foget render space, and even if both were just at DV rez (the 6 hour shoot was HD) I would have needed about 1.2T.
Just for the capture. As a typical example, triple it for render & scratch space.
The 20 hour project was all in native DV and required about 700-800G, including render & scratch space, so I suppose you might be able to work with about 2-2.5 hours of content total with a 120G drive. I think you could just straight capture maybe 6 hours on it. Oh, and two of the drives for that project were Maxtor 5400 RPM drives and they captured and rendered just as well as the WD SE drives used. The only time you could see a difference was when transferring between drives.