1. Motherboard - Socket 478 with an Intel chipset Never mind, missed that you were going with Tualatin
2. NVidia card - (Going with Voodoo2 ?) GeForce 4 Ti
3. Sound card - Soundblaster Live or Audigy 2 ZS - (Go PCI)
4. RAM - 512 MB

Note that unless you have two voodoo 2s laying around, expect to pay way too much for them....

Are you set on using a Tualatin processor while simultaneously requiring an ISA sound card? In that case, you can opt for a motherboard featuring the VIA 694T chipset. If you are unable to find one, a 694X chipset motherboard is also a viable option, though it may require you to make some minor physical modifications to both the CPU socket and the processor itself.

My recommendations are as follows:
Processor: Tualatin Pentium III-S
Motherboard: Based on the VIA 694T chipset (or 694X—modification required)
Memory: 512MB
Graphics Card: GeForce 4 Ti series (if Direct3D 9 support is required, the FX series is also an option)
Sound Card: Creative AWE64 Gold

I don't quite understand the rationale behind using an ISA sound card—is it for DOS compatibility? If you do not specifically require that capability, using an 815EPT motherboard paired with a PCI sound card would be a far better choice.

Before making any recommendations it would be good to know the specific goal(s) for the build.

Are there any performance goals for such a system? What specific games are you wanting to use it with?

For example, if the intent is to just use Win98 and Windows games, then an ISA sound card isn't really needed. There are PCI sound cards that would be preferable.

If the goal is to cover both DOS era and Win 9x era, especially if running native DOS, then there is more of a reason to have an ISA sound card (or two).

I would say Geforce 2 Ti and Aureal Vortex 2 would be a good match.

Thanks for all the recommendations! Yes, I do intend to do quite a bit of DOS gaming and would like this computer to handle DOS games with full sound support as well as Windows 98 games.

Do you have a budget?
You want a Voodoo 2 SLI and if you already not have those you already looking to spend 250-300€ at minimum for those alone. Then you might as well go and get an Aureal Vortex 2 too.