The 512 meg max is a limitation of the motherboard and not the BIOS.  No BIOS update is going to get around that limitation.  Here is the BIOS history for your model for your reference......

 

DELL COMPUTER CORPORATION
BIOS RELEASE NOTES
Copyright 1997 - 2002 Dell Computer Corporation.  All rights reserved.

System:        Dimension 4100
Version:       A11
Release Date:  06/19/2002

1. Addressed Audigy sound card "High pitched squealing" problem. 
********************************************************************

System:        Dimension 4100
Version:       A10
Release Date:  09/19/2001

1. It now only takes one key press or mouse click to resume from standby.
   Previous versions took two key presses or mouse clicks.
2. Windows XP SLP support added.
********************************************************************

System:        Dimension 4100
Version:       A08
Release Date:  03/30/2001

1. BIOS update code for Pentium III B stepping processors
********************************************************************

System:        Dimension 4100
Version:       A07
Release Date:  02/22/2001

1. Added support for Pentium III D0 stepping processors
********************************************************************

System:        Dimension 4100
Version:       A06
Release Date:  12/01/2000

1. Corrected booting from CD-ROM with IBM DOS (ver 7.1) boot image.
2. Fixed the issue of 766MHz Celeron® processor speed being displayed as 800MHz.
3. Fixed to correctly display POST error messages.
4. Faster resume from S4.
********************************************************************

System:        Dimension 4100
Version:       A05
Release Date:  09/22/2000

1. Fixed USB keyboard to work properly when Windows 98 SE shuts down and
   restarts in MS-DOS mode.
********************************************************************

System:        Dimension 4100
Version:       A04
Release Date:  08/30/2000

1. Reset patch installed to over come possible system hang at POST time.
2. Fixed keyboard buffer causing hanging condition if ESC key was pressed
   before booting to Windows 2000.
********************************************************************

System:        Dimension 4100
Version:       A03
Release Date:  08/18/2000

1. Fixed Windows 98 shutdown problem.
********************************************************************

System:        Dimension 4100
Version:       A02
Release Date:  07/14/2000

1. Improved POST boot time and hard disk detection.
2. Setup changes for Audio, Hard Disk size display, help text, AGP speed and
   CPU speed display.
3. Modified USB code to resolve a possible Windows install hang.
4. Fixed false POST errors for keyboard, CMOS, and audio.
5. Fixed possible Windows shutdown issue.
6. Fixed system hang from APM standby when USB Legacy was disabled.
7. Added support for more Pentium® III processors.
8. Fixed SMBIOS information for memory structures.
9. Fixed mouse detection in Windows 2000.
********************************************************************

System:        Dimension 4100
Version:       A01
Release Date:  06/28/2000

1. Intial Production release.
********************************************************************

Your welcome. I'm more than glad to have been of assistance.

 

As for using the 512 module...... that's a good question as I don't ever recall anyone trying it.  If it were me I would give it a try and see what happens.  The worse case scenario I suspect would be the system wouldn't boot up successfully with it installed.  There maybe other risks I am not aware of such as damaging the module, but then again we see a lot of postings in the forums here where people install incompatible memory modules and the system survived.