for diskspace:
df -h to see the diskspace free using human readable numbers.
du -h to see the diskspace used using human readable numbers.
for memory:
cat /proc/meminfo
free -m
top
vmstat
for cpu:
cat /proc/cpuinfo
and also cpu usage is shown with "top"
for eth:
ifconfig
and to view all this stuff at once you can get a nice little program called gkrellm that can shown this and many more
