And it was a BIOS issue. Upgrading to the A04 BIOS cured the problem and the system booted up normally with the AIW card installed. I'm installing drivers and software as I type this. Sorry for the false alarm, but the situation might possibly help someone else out further down the road. Thanks.  
Yes, I know that Windows XP is well past end-of-life. But I've always known that there was life beyond the grave. I'm doing analog video (VHS/8mm/Hi8) capturing on the side. As it turns out, the best hardware and software for this work was built and written for AGP slots and Windows XP...and Windows Vista (and beyond) broke the necessary drivers. I took the trouble to find a refurbished computer, Dimension 8250, with a legitimate copy of Windows XP Pro (SP3) and AGP. Now, I'm wanting to install a New Old Stock ATI All-In-Wonder 8500 video card in the AGP slot. I'm having problems. I've been using the system for a month capturing through USB with no trouble. A couple of minor mods; I installed a PCI SATA card and an internal 2 TB HDD, then yanked one of the DVD-RW drives and installed a hot-swap bay for capture files (a lossless analog video capture at full resolution can run over 1.5 GB per minute). The add-ons are working fine. Then, I tried to install the A-I-W. I get the Dell splash screen, so I know the card isn't DOA. But the splash screen goes about 80% of the way through the progress bar, then hangs. Power it off and back on; same thing. Power it off and back on again and I get the error message, "Alert! Previous attempts at booting this system have failed at checkpoint [Vini]. For help in resolving this problem, please note this checkpoint and contact Dell Technical Support." Well, I think it's about a decade too late for that last.... I did see another message on these same forums about clearing the NVRAM, so I followed those instructions...power down; unplug; pull the coin battery on the motherboard; hold the power button down for a minute to dissipate any residual energy...no help. However, when I remove the AIW card and reinstall the original NVidia card, the system boots. I've done everything I know to do to remove the old drivers; I deleted the video card in Device Manager, checked to see that the drivers were uninstalled, then shut down the system and again tried the AIW. Still no luck. I'm unable to get into BIOS at all with the AIW card in place; the computer hangs before it accepts the keypress to enter Setup. I can get into BIOS with the NVidia card installed, but I don't see any pertinent settings. The error screen up now says, "Phoenix ROM BIOS PLUS Version 1.10 A01". Would an updated BIOS help my situation? Anyhow, I really do want to get this hardware combination working; it would be perfect as a capture machine (air-gapped, of course...I don't want it on any networks). Again, Windows 8 or 10 or anything Vista and later is simply not a possibility; the hardware is useless without XP. (Or 2000, or Win98). It feels like I'm 98% of the way there...any suggestions to put me over the top?  
Dimension 8250 Not Booting; Stops at Checkpoint Vini