It better be. I just bought it. It says 20GB on the label.
New or used? If BIOS reports 8.4GB, that's all it will ever see unless an update fixes it. I suspect that's it because BIOS says 8.4, not 8.0.
Here's a link to the limitations:
BIOS LimitationsIssues fixed in A06 revision of the Dell Inspiron 3000 BIOS
1. Disable DIM LCD mode when Power Management is set to "Disabled" in the BIOS
2. Dell QuietKey keyboard did not work after resume from Save 2 Ram.
3. System could hang when installing Win98 onto a hard drive that did not have an existing partition if the user booted the system from the Win98 CD.
4. ZIP drive disappears after the system is restarted.
Flash A06 and pray!