That completely depends on where you are at and what platform you want to sell it on.

You can look up sold prices on eBay for similar systems and even the components by themselves to get an idea.

That being said, I've seen a range of probably $250 to $500 asking price on eBay for a system like that.

The valuable thing here is the Voodoo 3, which depending on the model can go for $150 or even more on ebay. So that might push the overall price of the PC up a bit. But those old Dell PCs don't tend to move that fast on ebay, and you have to get a special adapter if you want to change the PSU because they don't use the ATX standard wiring.

You would probably make more money selling the voodoo 3 by itself and the machine with some common AGP 2x graphics card (if you have one handy). The machine without graphics isn't worth much.