www.software4students.co.uk
I use these, they are pretty good and you can download instantly, cheapest place you will find office.
In regards to your question, I have not come across a pre-built machine that has a media disk for bundled promotional software, usually the builder has a special licence which allows them to install a certain version onto the machines.
You could try sending it back and asking them to install it for you and say that you purchased it for office but needed the 4gb of ram, however when you installed to the supplied 64-bit version of windows office was no longer available and could they please try and recover office onto the 64-bit version.