I've been using a KT333 board (ECS K7VTA3 v6) for many years as my fastest board that can run 3.3v cards. It works fine with AGP 8x cards too. Some KT333 boards don't support 3.3v though. There are different revisions.

VIA's AGP was not exactly perfect. There are compatibility issues with some cards. Radeon 8500 needed some BIOS updates back in the day for example for VIA boards.

Unplugging the power is always a good idea if you are going to add or remove hardware. An ATX motherboard is otherwise always powered to some degree. You don't want to accidentally trigger it to fully power on while hardware is halfway seated.

Intel i815 or SIS 645DX.

So let's see-- Here's my research results on the three chipsets--

• Intel i815 --> Probably the most compatible chipset but limited to a PIII-S 1.4 GHz Tualatin with the "B" stepping and 1.0Ghz Coppermine with the "A" stepping
• SIS 645DX --> Supports P4 Northwood 3.06 GHz CPUs and has good AGP compatibility but only supports single channel memory, which can cause 5% to 30% performance drop from the best case
• KT333 --> Supports an Athlon XP 2800+ Thunderbird or a Athlon XP 3000+ Barton which are likely the fastest here, but comes with the usual suite of Via AGP & PCI compatibility quirks

Does that look pretty accurate?

douglar wrote on 2026-06-17, 18:28:

So let's see-- Here's my research results on the three chipsets-- […]
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So let's see-- Here's my research results on the three chipsets--

• Intel i815 --> Probably the most compatible chipset but limited to a PIII-S 1.4 GHz Tualatin with the "B" stepping and 1.0Ghz Coppermine with the "A" stepping
• SIS 645DX --> Supports P4 Northwood 3.06 GHz CPUs and has good AGP compatibility but only supports single channel memory, which can cause 5% to 30% performance drop from the best case
• KT333 --> Supports an Athlon XP 2800+ Thunderbird or a Athlon XP 3000+ Barton which are likely the fastest here, but comes with the usual suite of Via AGP & PCI compatibility quirks

Does that look pretty accurate?

There's a thread dedicated to this, it's worth a look. Socket 423/478 Motherboards with Universal AGP Slot

KT333 still has pretty shaky AGP compatibility, so it's meh. Performance wise, it's the best option though. SIS 645DX just don't have good motherboards with plenty of overclocking options. i815 is the weakest one, but there are couple boards which also have ISA slot via third party bridge chip (compatibility may vary).

There is also SiS 650/651, some boards are 3.3V compatible. Regarding OC, all the 3.3V P4 compatible boards I had maxed out at ~150 MHz FSB... so not much to see here. Also VIA P4X266(A) and P4M266(A) are worth mentioning.

When speaking of widest compatibility range, I would add AGP Pro to the table... and that limits the chipset selection significantly. With this in mind KT333 would be the best, for example the mentioned Asus A7V333 has AGP Pro. The compatibility issues are to be expected mostly with late Radeon cards with Rialto bridge. These cards need significantly faster CPU anyway, so no big deal imho.

815 is solid for sure. It's just not that interesting unless playing with Tualatin is still exciting.

I ran VIA boards in 2000-2002 so have some affinity for them. I even find rediscovering the problems entertaining.

I did not pay much attention to SiS after using their Socket 3 and 7 parts.

Here the steps I'm following to get the best compatibility with the KT333 chipset:

System:
1) 09/10/2004 BIOS on the A7V333
2) Setting Primary VGA Card = AGP in the BIOS
3) Assign IRQ For VGA Enabled
4) Gentle memory settings

Video Card:
1) Disconnecting the power cable when swapping cards to avoid VGA BIOS initialization issues
2) Put the Latest VGA BIOS on cards if issues show up
3) Manually set the AGP Speed in the BIOS if there are issues
4) Avoid Interrupt sharing

Base OS before installing Video Driver:
1) Win98SE
2) Via 4-1 driver Version 4.43
3) KB260067 - AMD CPU "Large Page" Patch
4) Direct X 9.0c

Oh, one small point - none of these options will be fully compatible with really old AGP 1x/2x cards physically if they have this stupid latch on the slot.

The Serpent Rider wrote on 2026-06-18, 19:28:

Oh, one small point - none of these options will be fully compatible with really old AGP 1x/2x cards physically if they have this stupid latch on the slot.

I don't have the latch. I have AGP Pro Universal.

The Serpent Rider wrote on 2026-06-17, 20:44:

KT333 still has pretty shaky AGP compatibility, so it's meh.

How so? I have a couple of MSI KT3 Ultra 2's and they run everything I throw at them - from a trident blade 3d to a X1950 AGP. SiS, 3dfx, S3, nvidia, ATi, everything just worked. I've also used a Soltek KT333 (can't remember the model) with identical results. The only KT333 I've had issues with is an Asus A7V333 that did not like radeon 7500 and 8500 cards and a Gigabyte GA-7VRXP that didn't like some cards with PCI-E to AGP bridge chips.

The Serpent Rider wrote on 2026-06-17, 20:44:

i815 is the weakest one, but there are couple boards which also have ISA slot via third party bridge chip (compatibility may vary).

I've never seen an i815 board with an ISA slot apart from some oddball industrial jobs that lack an AGP slot - I didn't even know that was a thing. In fact I'm pretty sure all 8xx chipsets don't support ISA natively and require a PCI to ISA bridge chip.

The VIA 693T chipset on the other hand has excellent AGP support (universal), great memory subsystem performance, supports ISA and has Tualatin support. Same for the VIA VT8633 (Apollo Pro 266).