Hi Mark,
I have a Dell PE 2950 under Proxmox 1.x from quite a long time now. I did nothing peculiar, it worked out of the box.
Here is the NIC model from lspci :
05:00.0 Ethernet controller: Broadcom Corporation NetXtreme II BCM5708 Gigabit Ethernet (rev 12)
At least, eth0 and eth1, the two ports I use are working correctly...
Alain