Interesting Problem. I have a much older Abit KT7-raid with KT133 chipset and that had no problems with a
9800XT. I think I even had a HD3650 running on it at one point.

I hope Your power supply has a powerful +5v rail, as the Athlon XP chips and earlier all run off the 5v rail as
opposed to 12v rail on P4's Athlon 64 and later. More than 30A would be needed.
You didn't mention what OS You have installed either, although I assume Windows XP?

Yeah, this issue seems to be specifically a KT400 issue, alongside contemporary chipsets supporting AGP 8X (SiS 648, P4X400). I do have a KT7-RAID and an Asus A7V266 (KT266) but both need recapped badly -- actually the A7V266 might not work at all, I just rescued it from a PC that was in someone's garage for years I think it's rusted out a bit too. I am aware of the amperage needs for an AXP, the PSU in the system has 34A on +5V. I have another I could try but it is right at 30A.

One option I am looking at right now is grabbing my other 9700 out of my Power Mac G5. It is a flashed PC card and might be a slightly later revision. I'll have to see if the other card can take the Mac ROM flash though... so it's a bit complicated and that's why I haven't gone through with it yet. Going to hold off but keep it in the back of my head.

And sorry, yeah, this is WinXP. I was up way too late last night troubleshooting so my post is a bit of a ramble and I forgot. 😀

Known VIA/ATI AGP issue, I had a similar problem on a KT600 board. Just Force it to run at AGP 4x via BIOS and SmartGART and you will lose nothing.

That's the problem though, the BIOS does not seem to allow me to change the AGP speed*. It is greyed out and just stays at "AGP 8X". Can I tweak SmartGART settings without the driver loaded? I can get into Safe Mode for registry changes or whatnot

*Despite the manual saying I can.

1AGP Capability [8X Mode]
2This motherboard supports the AGP 8X interface that enables enhanced graphics performance with high bandwidth speeds up to 2.12GB/s. AGP8X-X is backward-compatible. When set to [1X Mode], the AGP interface only provides a peak datathroughput of 266MB/s even if you are using an AGP 8X card.
3Configuration options: [Auto] [1X Mode] [2X Mode] [4X Mode] [8X Mode]

The following options are described too but don't exist. I wonder why? Was the manual based on an internal version with more options?

1AGP Drive Strength [Auto]
2Configuration options: [Auto] [Manual]
3
4AGP Drive N Control [E]
5Configuration options: [0][1][2][3][4][5][6][7][8][9][A][B][C][D][E][F]
6
7AGP Drive P Control [F]
8Configuration options: [0][1][2][3][4][5][6][7][8][9][A][B][C][D][E][F]

The 9700 was incompatible with some Mainboards, its fixed in 96xx and 98xx Series

Reboot into Safe Mode, run ATI Control Panel, under SmartGART set AGP speed to 4x, reboot.

Well, I got somewhere but I'm not exactly sure how.

I realized that I might be able to adjust the AGP Capability in the BIOS if I wasn't actively using an AGP card, so I swapped in a PCI card and was able to actually change it from Auto to 4X. Unfortunately, putting the 9700 back in still made it say 8X. However this swap had the weird side effect of Windows suddenly being able to boot up normally. Although 3D acceleration caused the system to lock up and all tools reported that the 9700 was running at 8X. Well either way, I tried to change the SmartGART setting only to find that the 10.2 "Legacy" drivers don't seem to have that (and also I couldn't get into CCC at all in Safe Mode before because it saw the driver wasn't loaded...). So I removed them and jumped back to the next driver I had laying around, which was the final Omega drivers from 2007 (Catalyst 7.12 based). That version does have the SmartGART control panel tab and I was able to set it to 4X. Now everything seems to be working okay. I could probably also use PowerStrip or something to force 4X if I need a newer driver, I suppose.

I dunno how long term stability will be, but I'll try it for a bit. I'll eventually remove the overall issue and get a 9800 instead, maybe whenever I grab a Barton 3200+ 😀

Try out Catalyst 7.11 too. Catalyst 7.12 and beyond have changes to OpenGL and Direct3D that aren't really in the best interests of your old games. KOTOR 1/2 won't render some things (yet again) for example because they drop some old OpenGL extensions. From what I gather they overhauled the OpenGL ICD and 7.12 was its debut. D3D8 problems arise too like bump mapping in Republic Commando being broken.