the latest version of wordpress provided by fantastico is 2.3.3
You can refer
http://netenberg.com/fantastico_scripts.php for the fantastico supported scripts version.
Wordpress 2.5 is not yet available with fantastico installation scripts.
You can try installing Wordpress 2.5 manually, If you need any help
then please open support ticket on Help desk.
EUKMatthew
http://eukhost.com