Just note. With the OEM version, once it is 'activated' on a particular computer, it can not be transfered to a different computer. So, if you sell your intel iMac and buy an intel iBook, you will have to buy another copy of XP.
I still think going OEM is the best deal. In the above scenario, you are out no more $$ than the cost of XP Retail and you can run XP on two computers.