"Will XP do a good job of detecting the changed hardware?" Probably not.
If you search Google Groups for "XP upgrade motherboard", you may get a few
hits. (A few tens of thousands, really.)
The common fix is to do a repair installation of XP:
http://support.microsoft.com/default.aspx?scid=kb;en-us;315341
"How to perform an in-place upgrade (reinstallation) of Windows XP"
(In brief: don't let the new machine boot into Windows. Change the boot
order to start from the CD drive first. Boot from the XP CD. Choose to
install, rather than going to the Repair Console. Under install, choose to
repair the existing installation rather than making a new one.) This will
preserve most installed programs and settings, but it will eliminate
everything installed using Windows Update. Some applications may need to be
re-installed as well, but not most. Windows activation will also be
required.
The above assumes that you have a retail XP CD.
If you want to use an OEM CD that came with the Dell, you'll probably have
trouble. License issues aside, the XP version may be locked to install only
on a machine whose BIOS identifies it as a Dell.
Also, you may have some trouble swapping an industry-standard mainboard into
your 4100:
http://support.dell.com/support/edocs/systems/dzuul/techovu.htm
Take particular note of the "auxiliary power connector" and the "control
panel connector". These are Dell proprietary features that are incompatible
with standard motherboards. Modifying things to fit may be possible, but it
wouldn't be worth the effort for me. Your mileage may vary.
I'd suggest starting afresh with an Antec case (
www.antec.com), although
there are serviceable generic Asian ones available at lower prices. (The
power supplies in the generics won't be as good as an Antec supply, though.)
I'm using a Super Lanboy, even though I didn't want the window, and I don't
go to LAN parties.
Address scrambled. Replace nkbob with bobkn.