I'd have to say debian. For 2 reasons:
1. It doesn't install a GUI by default, and doesn't hog resources.
2. You can get security updates off the net pretty easily.
I wouldn't stick Mandrake or something on that machine, as it would install httpd, bind, pcmciacs and loads of other daemons you don't need, each taking up memory and cpu time that you don't have.
You want it trimmed down, the only 'services' you want running should be ssh and ftpd (and maybe webmin if you want to go that way, it can simplify configuration if you don't know what you're doing)
i.e. my debian server has these processes:
Code:
richard@files:~$ ps ax
PID TTY STAT TIME COMMAND
1 ? S 0:06 init [2]
2 ? SW 0:00 [keventd]
3 ? SW 0:00 [kapmd]
4 ? SWN 0:00 [ksoftirqd_CPU0]
5 ? SW 0:09 [kswapd]
6 ? SW 0:04 [bdflush]
7 ? SW 0:10 [kupdated]
9 ? SW 0:09 [kjournald]
78 ? SW 0:00 [kjournald]
104 ? SW 0:00 [eth0]
172 ? S 0:00 /sbin/syslogd
175 ? S 0:00 /sbin/klogd
184 ? S 0:00 /usr/sbin/sshd
195 ? S 0:00 /usr/sbin/atd
198 ? S 0:00 /usr/sbin/cron
200 ? S 0:00 /usr/sbin/xinetd -reuse
202 tty1 S 0:00 /sbin/getty 38400 tty1
203 tty2 S 0:00 /sbin/getty 38400 tty2
204 tty3 S 0:00 /sbin/getty 38400 tty3
205 tty4 S 0:00 /sbin/getty 38400 tty4
206 tty5 S 0:00 /sbin/getty 38400 tty5
207 tty6 S 0:00 /sbin/getty 38400 tty6
216 ? S 0:02 nmbd -a
1473 ? S 0:00 ./server -auto
1477 ? RN 5145:21 ./think -n 0
3208 ? S 0:03 smbd
4237 ? S 0:00 smbd
4836 ? S 0:00 /usr/sbin/sshd
4838 ? S 0:00 /usr/sbin/sshd
4839 pts/1 S 0:00 -bash
You want to keep it trimmed down on a system like that, so I would also avoid installing X on it, you could always tunnel through ssh if you need a graphical app anyway.