[64 bit] installed 32-bit xubuntu on 64-bit dual-core.. should/can I upgrade?

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http://ubuntuforums.org – My question has a few parts: I just installed the 32-bit version of xubuntu a couple days ago on a Dell optiplex GX620 with 486 MB SDRAM (i know) and a 3 GHz dual-core processor with 2 MB L2 cache memory. I plan on adding 2 GB of RAM (this should be easy right?), bringing the specs up to just under 2.5 GB memory. I've heard that some of the advantages of 64-bit computing come with a larger available memory space, starting at 3 GB. Is there any reason to install 64-bit xubuntu? If so, is it possible to upgrade from the 32-bit version I currently have installed without having to reinstall eve (Hardware)