the dell system cdrom`s actually check the bios to make sure that it is the corrrect bios, however just because you have a dell machine does not mean that it will work on all dell machines. dell incorperated a series number for each product to avoid piracy. the dell 8300 series desktop will be diferent from the dell 2350 desktop or 8500 insperon laptop.
however, dell allso assigned a product OEM sticker for the os if you had to format the drive and copy the I386 files over to install. however you MUST regester it . The product Id is located on the right side of the dell system.

I would go with a retail copy of xp because dell incorperated their software into the os as well. with a retail copy you will not get anything that is OEM specific.

Just a tech`s 2 cents.