Installing Windows 7 Pro X64 From Digital-driver With Windows 7 Pro OA X86 Key?

I have a HP Pro 3300 Series MT i3-2100 PC. It came pre-installed with Windows 7 Pro 32-bit. On the product key sticker it states 'Windows 7 Pro OA'. I have checked the 'Performance Information and Tools' section in Windows and it states that my PC is 64-bit capable.My PC was bundled with 4 HP Recovery Manager discs, but these all state 'This disc contains 32-bit software'. I want to upgrade my RAM to 8GB so would like to install a 64-bit version of Windows 7.I would like to know if it is possible for me to do a clean install of Windows 7 Pro 64-bit downloaded from the Digital River website and activate it with my 'Windows 7 Pro OA' product key. I am aware that a product key can be used for both 32-bit and 64-bit installations as long as they are the same version of Windows 7, but can a OEM product key of Windows 7 Pro activate a copy of Windows 7 Pro downloaded from the Digital River site? (which I guess would be the retail version.)Also, my computer is partitioned into 3 drives. C: - containing the Windows operating system etc. D: - An empty drive of about 200gb, and O: - HP Recovery Drive.If I am doing a clean install of Windows 7, is it possible to retain the partitions and only wipe the C: drive during re-installation, allowing me to move any data I wish to keep to the D: drive in advance, or will it all be destroyed and I'll need to move all data to be retained onto external media beforehand?