Hello hqasem,
Were you able to get this to work? I want to do the same thing with my Poweredge 400 SC. 

1. Attach the USB Flash drive and power on the server Boot the system to setup by pressing F2. From the main page arrow down to Integrated Devices and press Enter. 2. Verify that the USB selection is set to On, with bios support. Arrow down to the option and use the left / right arrow keys to change the setting. Press ESC once to return to the main page. 3. Verify that the USB Flash Drive emulation type is set to Auto. Arrow down to the option and use the left / right arrow keys to adjust the setting. Press Alt + B to save and exit the changes. 4. Press F2 during POST to enter setup. Verify that in Boot Sequence, Hard Disk drive C: is listed and has a check mark next to it. 5. Once in setup arrow down to Hard-Disk Drive Sequence and press Enter. Locate the Hard-disk-emulated USB flash drive and press the - key to move it to the top of the Hard Drive Sequence. Press Alt + B to save changes and exit. The system will reboot. If no CDROM/Floppy installed the USB Flash drive should be bootable.

I was able to do this on a PowerEdge 750. It has similar BIOS options as the 800.

I set my "USB Flash Drive Emulation Type" to "Hard drive" -- not "Auto" as hqasem says.

Then when you look in the Boot Sequence menu, the hard drive listed there is not actually a hard drive. It's the USB device that is emulating a hard drive. 

Set that as your boot option and you should be good to go. Hit ESC and then "Save and Exit" and the computer should boot from you USB.

Source: http://www.itfromscratch.com/how-to-boot-from-usb-on-a-dell-poweredge-750/