Upgrade a 64 bit Dell OEM Vista Business to Ultimate.

I need (want) media center.

How do I do this ?

Can I buy an OEM upgrade from Dell ?   Is there such a thing ? I talked to Anytime Upgrade at Microsoft and they told me it only applies to 32 bit.

Can a retail upgrade work on an OEM installation.

I have no problem re-installing as my machines are very simple but I want to remain legit.