I have Windows Server 2003 32bit
As per Tech Specs, you can install Media Server 4.5 on Microsoft® Windows Server® 2008 R2 64 bit.
Check the below link for details.
[Moved the discussion to Adobe Media Server forum]
I am no adobe media server expert but you will be able to purchase it. What i am not sure off is that whether you will be able to install it on a Win Server 2003 32 Bit or not as the system requirements does not meet.
If you want you can still try to install a trial version of Media Server 5 starter edition and check if it works fine. If it does, you can purchase it then.