Any Broadcom 5709 (dual port) or Intel 9402, or Intel E1G42 should work fine. If you want it covered by your server's warranty (if it still has one), you can purchase the card from Dell (be sure to buy one with a partnumber that is shown as 3 digits, a dash and then 4 digits (like 123-4567), or a 5 character part number. If you buy a 'software and peripherals' card, it's basically retail, which means 30 (or maybe 21) days best effort support from Dell, and then you have to go through the manufacturer (usually means sending it in and a week or so later you'll receive a replacement).
I need to add a dual-port 10/100/1000 NIC to my 2950.  I've got an open PCI Express slot, but am looking for a recommendation for a good NIC.  The server is running VMware ESX, so it needs to be supported.  It looks like it's got Broadcom internally, but I'd be happy with Intel as well.  Any suggestions? 
Need a dual port NIC for a 2950