First of all disable all the services you don't need (dunno which tools Mandrake has for this).
If you aint serving webpages, disable Apache.
If you ain't serving ftp, turn off ftpd.
If you don't want people to use your server as a mailserver (eg, to send mail *to*) turn off sendmail.
If you ain't *providing* cross-platform sharing turn of samba and any "r" services like portmapper and the like.
Disable telnetd and fingerd anyway, identd may be needed by some mailservers.
For any of these you do use that are started from (x)inetd, you can use "TCP Wrappers" to allow/deny access to these services from /etc/hosts.(deny|allow) for a more fine-grained control who may access these.
Then add rules to your firewall config to block access and log attempts.
Get some basic understanding of security issues from the UNIX Security Checklist v2.0
, same for Packet Filtering for Firewall Systems
. Since Mandrake is based on Red Hat you might also like Securing and Optimizing Linux (RedHat Edition)
or, more appropriate Securing Linux Mandrake
, or more general (but a wee bit older) Elementary security for your Linux box