Also be AWARE that CentOS 5.5 is and has been
unsupported for a long time
sense it went END OF LIFE on "2011-04-08" -- April 2011
Warning:
CentOS 5.5 has had NO security updates in almost TWO YEARS
CentOS 5.9 is the current in the older 5 series with CentOS 6.3 the current in the current 6 series