TLDR; I am looking for a workaround / solution to the Code 43 error when combining an Nvidia Nforce 3 250 chipset AGP board with an ATI AGP graphics card.
I would like to be able to use a dual / quad core with my motherboard and a 64 bit Windows OS (Windows 8.1 x64; I have used that OS with socket 775 boards and ATI AGP cards).

So I have been messing around and gaming a lot with my 3850 and 4670 AGP cards.
I mainly used a Asrock 4Core-Dual Sata2 for this.
I have also owned a similar Asrock socket 939 board, but that was seriously bottlenecking those graphics cards even with an X2 4800+
Probably also RAM bandwith bottlenecked.

Later I found out that Asrock had also made several boards that support even a Phenom X4 980 (unofficially) and 8gb DDR2.
So I started looking for this board and an ASRock ALiveDUAL-ESATA2.
A couple of days ago someone gave me a Asrock AM2NF3-VSTA board, fully functional with the latest 3.30 bios.
This board features an Nvidia Nforce 3 250 chipset... which is (part of) the problem with this board and the combination with ATI AGP cards.

Even with manually installing ATI graphics cards drivers through device manager, the cards will stay in "Code 43" mode.

Apparently, this is in Nvidia driver issue, because Nvidia never released a 64 GART driver.
Some say a workaround is to disable all but one CPU core.
I have tried this (Not possible in my bios) through Windows 8.1 startup settings... that had no effect.

I have found some topics here / online of people that are using this board without problems even with an older X64 Windows 10 (because not all Windows 10 versions have AGP support) and multi core cpu's.
So this board seems to work perfectly with X64 bit OS, multi core cpu and Nvidia(!) graphics cards.
Once you try Ati cards; no dice...

I have seen this code 43 before with an other combination of similar hardware (which I do not own anymore).
That was an ATI 3650 card in a socket 754 Shuttle SFF system with a 32 bit Windows 7.
I had solved those issues then by installing "Nvidia nForce PCI System Management" drivers from "Fernando's pecial Nforce drivers".
But then, again, that was a 32 bit OS and a single core cpu.

Edit: So I did get hardware acceleration / a working 3850 in 8.1 32 bit... by disabling one cpu core through MSCONFIG startup settings.
I could even install other Nvidia GART drivers... this resulted in a BSOD after installation... but those drivers were installed after a reboot (confirmed with device manager).
Going back to the dual core setting resulted in not being able to boot into Windows again with drivers included with Fernandos package (2008 Nvidia GART drivers); a BSOD every time.
Better stick to the included drivers released by Microsoft.

Maybe this issue could be fixed by doing some editing and adding some device manager entries.
I remember fixing a similiar issue with an external GPU, by adding more 32 bit adressing space.