In this context, yes, 'OEM' and 'Branded' equate to the same thing - a version of WinXP that's specifically targetted to a particular brand of hardware. At the end of the day the underlying software (i.e. Windows) is the same. but the installer may may checks for specific identifiers in the hardware that denote the brand of the machine, and may also install brand-specific software such as drivers, applications, etc.
Common examples would where people like Dell ship OEM versions of Windows that include drivers for hardware they sell (e.g. the specific brand of video chipsets they use in their machines) while excluding others (they don't need to ship, say, Nvidia drivers if they only sell ATI cards), as well as support applications (e.g. 'phone-home'-type apps that help their customer support department troubleshoot client problems).
Of course, that's not to say that you can't install additional drivers, or remove the cruft they do install, but it also delves into a murky legal area where that copy of Windows was licensed to go with a specific hardware model and installing it somewhere else violates the license agreement.
So whoever's telling you to get an OEM version is either misleading you (probably think they're helping by saving you a few $$s), or they don't understand what you're trying to do here.
At the end of the day, to install Windows on a Mac you need a full retail copy of the OS in order to comply with the license terms.