Wow you guys are channeling me, it seems.
tkam is right, just get someting in the price range. I have the 520HX and I don't seem to remember if it has 2 6pin plugs because most new video cards require 2.
If you are concerned with power/noise, I'd recommend the GTS 250. Granted, it's not the most powerful but it'll run oblivion fine. If you want something better, I'd try GTX 260 or 4870. One thing to watch for is Dell's terrible cases. Air flow in those tend to be terrible and may have some issues with GPU overheating.
For hard drives, if for file storage, I'd HIGHLY recommend the WD GP drives as Fungi said. They are dead silent, reliable, low power, and plenty fast for file archival. If you plan to run apps off it, look somewhere else. DO NOT look to Seagate or Maxtor. Seagate fell off the boat with the .10 HDs.
edit: saw that you were doing raid0. Probably means that you will be running apps/games. Ignore my WD GP advice.