When I built a Windows 98 machine on an even newer chipset, the 865, apart from the lack of AGP support for old cards, I really couldn't find any disadvantages.

If your expansion cards work with a Pentium 4, I agree, you could save quite some money.

Well, the SIS 645 for example, and even some Intel 845 motherboards have the same configuration AGP slot as the 815EP.

Having said that, the 815EP system I'm after isn't that expensive. The seller is asking roughly $30. It's got a 1GHz CPU, TNT2 M64 Graphics card and a Sound Blaster CT4830 in it as well.

Some of those early SIS and VIA P4 boards are 3DFX compatible but in general are pretty poor performance compared to other P4 boards yet still very competitive compared to most P3 builds. If you could get both cheaply it would be best to buy both but seeing how P3 gear is getting to be rare in some parts of the world these days I would go with that.

I only own one 3DFX card. It's a Voodoo 3 3000. I currently use it in a Socket 370 440BX based board. Would it work any better in an 815EP Motherboard?

A 478 P4 Motherboard and CPU costs under $10 here to give you an idea. I have plenty DDR and PC133 RAM already, so that's not an issue.

As far as I know, Intel 845 is 1.5V only... so not not compatible with AGP Voodoo3, Riva TNT, some Kyro 2, etc.

On the other hand SiS 645 can run all AGP video cards (in theory at least). One of the not compatible VGAs is Voodoo5 6000 - not a problem for most people I guess. 🤣 General performance is not bad at all. More or less the same as i845 boards or RDRAM based P4 systems.

Most (if not all) SiS 645 boards support FSB 533 with correct AGP and PCI dividers. So usually it is no problem to run P4 2.8 Northwood FSB 533. Asus P4S333 works well even with P4 3.06 (HT not available though).