Slot 4 or 5, stick your modem in the other one (4 or 5). Sound Card Slot 3 (like you have it) and then I think Slot 2 should be left open if possible. You'll most likely only use your modem or your NIC one at a time, so they shouldn't conflict.
It depends upon your soundcard and whether or not you are using the ATA/100 slot.
I cut and paste this a lot:
PCI slot 1 shares with the AGP.
PCI slot 2 shares with the onboard ATA/100 controller.
PCI slot 3 is independent--the only one.
PCI slot 4&5 share IRQ with each other and with the onboard USB.
I have my NIC in slot 3, my SBLive in slot 4, my Firewire card in slot 5. Nothing in slots 1 or 2--I am using an ATA/100 drive.
My system is running very stable. If you have a SBLive card, do not install liveware.
I have switched the NIC and SBLive card between slots 3 and 4 and I have not noticed any stability difference. The above was the last one I tried and have left it that way. I was having other problems related to the video driver and was trying everything.
In the end you will have to try it yourself on your system to know for sure what will and will not work.