Sounds like you have already purchased Windows Ultimate N version already.
The "N" stands for WMP, DVDmaker, Media Center, IE8 has been removed by M$. Its the European version. You can download the missing programs from the Windows website however. Therefore you must live in Europe somewhere.
Since you have 4GB of RAM, go ahead and install the x64 version. It will utilize the full potential of the RAM and your CPU.
I would highly recommend that you do a clean install of your x64 OS. Especially if the Home Premium version is x86 (32 bit) since you cannot do an "uprgrade" from x86 to x64 OS.
P.S.
From Wikipedia: As with previous versions of Windows, an N version, which does not come with
Windows Media Player, has been released in Europe, but only for sale directly from Microsoft sales websites and select others.
So I was wrong on some of the above on my post. Ah well...