well the boards I use for testing have AGP x4 only slots, they are the following:
ASUS A7V333-R Rev.1.02 with Bios 1017 Final
VIA Apollo KT333 chipset for Socket 462
1x Universal AGP Pro 50 x4 Slot
5x PCI 32 Bit Slots
the test witha Ralito based XFX GeForce 7600 GT 580M AGP has failed in AGP x4 mode on this board, the idea was to see if the Ralito would accept the AGP x4 bus.
The other board I use for testing is based on a Legacy Pentium4 Intel chipset & Design.
ASUS P4T Rev.1.04 With Bios 1007
Intel i850 Chipset for Socket 423
1x True AGP Pro 50 x4 Slot
5x PCI 32 Bit Slots
this board gave the same Anomalies with the Ralito based AGP x8 card, so my conclusion that it's not a chipset realted subject.
So Jan it would be great if you have a system that makes use of the Intel i850 or VIA Apollo KT333 chipset, then we can find out if it will really work.
the ASUS ATi Radeon X800 XT Platinum Edition AGP x8 works perfectly the same goes for the HIS ATi Radeon 9550 AGP x8 256MB DDR, Hercules ATi Radeon 9700 Pro AGP x8 128MBDDR, XFX NVIDIA GeForce 6800GT AGP x8 256MB GDDR3 as Leadtek NVIDIA GeForce 6800 AGP x8 128MB DDR and Gigabute NVIDIA GeForceFX 5700 AGP x8 128MB DDR.
All of those cards functioned without flaws in AGP x4 mode in both system setups.
So If a X1950 Pro AGP x8 works stable in AGP x4 in either setup which I have listed, then a X1950 XT should do the same, since they both have the same card design qua PCI-E to AGP layout.
My Thanks goes to all helping me study this nasty thing