Dennis is right, you can use the XML backup and import it in a new JIRA 5 64Bits instance. I just would like to give you a few more details about JIRA and Java versions.
In fact what define if JIRA will be 32 or 64 bits is the Java version, if you are using JIRA 4.x standalone, you can just install Java 64bits and edit your Java Home to start using JIRA over 64bits platform.
One important detail is that when using JIRA installer, it ships the Java bundled in it. In this case, the Java location is <JIRA_Install_directory>\jre
Aditionally to this, we have this documentation that has more details about upgrading JIRA.
I hope this information has helped.
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