PCI display adaptor stops working after adding 2nd pci card

I have a circa 1996 windows PC (pentium MMX 200 MHz, 81M ram, 4 PCI, 2
ISA slots, Award BIOS). A new installation of Win98 SE is installed.
The ATI display adaptor is located in 1 pci slot. The Soundblaster
16bit sound card is in an ISA slot. No other plugin cards. This works
fine.

When I add a 2nd pci card into another PCI slot the machine stops
booting. The display stays dark and the Harddrives stop spinning.
When I remove the added card the machine returns to booting fine. I
have replaced the new card and the problem occurs with the replacement
card as well.

I have tried putting the PCI cards in different slots but the problem
continues.

The new PCI card is a Netgear wireless PCI 802.11g network adapter
card.

I thought there might be an IRQ conflict but the display adapter
doesn't seem to use an IRQ. In the device manager-resource tab it
doesn't report any IRQ.

I tried turning off IRQ steering but it was no help.

Any ideas?