Download the CentOS LiveDVD from here: http://linux.dell.com/files/openmanage-contributions/om-firmware-live/
next, copy it to a usb drive with dd:
sudo dd if=<path to CentOS iso> of=/dev/<my usb drive> bs=4096
This image should have all the latest firmware built into it. I used this to update BMC firmware for the Dell R900.
There is a blog by Dell about this procedure as well, shown here: