It is highly recommended to update SAP GUI to the latest versions due to the Support Deadlines.
SAP GUI for Windows Support Deadlines
6.20: The restricted support applies since 30th of September 2005 (see SAP Security Note 929300 for more information). Support for SAP GUI for Windows 6. 20 will end on 31st of December 2010.
7.10: Up to April 12th, 2011.
7.20: Up to April 9th, 2013.
Old versions such as 6.4 may contain vulnerabilities that cannot be patched because support is finished. If you are interested what kind of vulnerabilities exist in your installation and how they can harm your company environment you can try free online scanner ERPSCAN.
Additional information: Support Note 147519 – Maintenance strategy/ deadlines ‘SAP GUI’