It is during the post screen. Generally it is either <del> or <F2> while it is doing the memory test. In your case, Dells will want you to push <F2> to get into the BIOS and <F12> to just get into the boot menu.
Just set the drive detection section for AUTO and it will find whatever you add HD-wise, it will then let you know when you change drives and will ask you to push F2 to save teh new configuration.