I've put four nics in lots of old computers, make sure you're not using isa nics! Also make sure the bios is set to plug and play mode. Or get a new mb with dual integrated nics and add two.
Thanks...:). Well, I have for intel nics (Intel pro 10/100 pci) on OptiPlex and only two are working. Seem to me, they have shared irqs.
Yep, they have shared irqs. I put four intel cards in one machine and only two are working ... depends on which two configured first.