Can one disable the onboard display adaptor in Win98SE?

I have a Dell Dimension 2400, running WinXP-Home. The PC has on-board
integrated video (Intel 82845G/GL... Graphics Controller).

I installed Win98SE on the PC for dual boot. [Dell does not support
Win98.]

I bought a PCI video card -- MadDog's nVidia GeForce FX5200 Plus, with
128MB of ram and has both a VGA and DVI-I connectors. I set the BIOS'
Primary Video Controller (under Integrated Devices) to "Auto" (it is
either Auto or Onboard).

I have installed the latest driver (from nVidia) for the video card.
And I am having problems with the monitor (CRT) -- it is stuck in VGA
because the nVidia Video Card "cannot find any free Memory Range
resources to use (Code 12)". The Conflicting Device List are all with
"System Board Extension for ACPI BIOS" in the following memory ranges

00000000 - 00FFFFFF
00000000 - 07FFFFFF
000C0000 - 000CF7FF
00000000 - 0001FFFF

And it won't let me change the Resource Settings for the video card
("This resource setting cannot be modified"). And I have no idea which
devices are occupying those memory ranges.

Any help would be appreciated.

TIA