The 64-bit Symantec Management Agent, like any 64-bit application, will be faster and capable of accessing more memory, though I wouldn't think you need more than 2GB RAM anyway. One main importance is the ability to gather and report on 64-bit inventory data, including custom inventories on 64-bit systems. For example, see the Warning at the beginning of this KB:
http://www.symantec.com/docs/HOWTO54431
You can enable the 64-bit plug-ins and they'll find the correct homes based on policy target and resource applicability.
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