Here are a few command line tools that will help you estimate your system usage and performance.
There is a lot more information on this in the man pages so take a look there also. If you dont know your average system load you will not be able to determine if the load is in a peaking state or if the host is able to handle more load.
Note that some commands might require installation of a few packages.

Below gives a nice simple performance view of a HDU. Ofcourse running this on a fully loaded system will lower the values. Do this verification just after installation. I usually only use it if I think the system is running slow and I want to see if the disk load is too high. And yes.., this is an old laptop so dont comment on the measurements.

# hdparm -gTt /dev/hda1
/dev/hda1:
geometry = 65535/16/63, sectors = 13671252, start = 63
Timing cached reads: 518 MB in 2.04 seconds = 254.40 MB/sec
Timing buffered disk reads: 66 MB in 3.00 seconds = 21.99 MB/sec

Below present a report on CPU usage. But even more useful is the I/O statistics for devices and partitions.
It is also able to display I/O for NFS but I dont have any mounted in this example.

# iostat
Linux 2.6.26-2-686 (testbox) 02/17/2010 _i686_

avg-cpu: %user %nice %system %iowait %steal %idle
30.61 0.00 3.57 4.62 0.00 61.20

Device: tps Blk_read/s Blk_wrtn/s Blk_read Blk_wrtn
hda 6.99 156.13 81.45 3618343 1887544
hda1 5.73 81.70 69.33 1893342 1606784
hda2 0.00 0.00 0.00 6 0
hda5 0.06 1.63 3.88 37765 89960
hda6 1.19 72.80 8.23 1687014 190800
hdc 2.30 533.78 0.00 12370344 0

Below will display a usefull compact overall report of the system state. This clearly indicate an overloaded host.
Some information is trivial. You can see that in many ways. Pick whatever you like best.

# vmstat
procs -----------memory---------- ---swap-- -----io---- -system-- ----cpu----
r b swpd free buff cache si so bi bo in cs us sy id wa
1 0 12128 215028 520 114336 1 2 346 40 369 1476 31 4 61 5

The description of each field from the man page is as following:
Procs
r: The number of processes waiting for run time.
b: The number of processes in uninterruptible sleep.
Memory
swpd: the amount of virtual memory used.
free: the amount of idle memory.
buff: the amount of memory used as buffers.
cache: the amount of memory used as cache.
inact: the amount of inactive memory. (-a option)
active: the amount of active memory. (-a option)
Swap
si: Amount of memory swapped in from disk (/s).
so: Amount of memory swapped to disk (/s).
IO
bi: Blocks received from a block device (blocks/s).
bo: Blocks sent to a block device (blocks/s).
System
in: The number of interrupts per second, including the clock.
cs: The number of context switches per second.
CPU
These are percentages of total CPU time.
us: Time spent running non-kernel code. (user time, including nice time)
sy: Time spent running kernel code. (system time)
id: Time spent idle. Prior to Linux 2.5.41, this includes IO-wait time.
wa: Time spent waiting for IO. Prior to Linux 2.5.41, included in idle.
st: Time stolen from a virtual machine. Prior to Linux 2.6.11, unknown.


Below is generating a rapport of CPU related information. Load for both applications and kernel, idle time etc.
This is very handy on a host with more CPUs installed. Unfortunately below is from a host with one CPU having only one core. Ill try to redo this later.

# mpstat
Linux 2.6.26-2-686 (testbox) 02/17/2010 _i686_

02:58:27 AM CPU %user %nice %sys %iowait %irq %soft %steal %idle intr/s
02:58:27 AM all 30.58 0.00 3.04 4.62 0.46 0.07 0.00 61.23 368.12


Below will display used and free memory. Though this rapport do not tell the whole truth. Some of the memory stated as used might not be. It might be reserved for buffers etc. The amount of free memory will always be rounded down.

# free -ml
total used free shared buffers cached
Mem: 472 354 117 0 10 240
Low: 472 354 117
High: 0 0 0
-/+ buffers/cache: 103 368
Swap: 1349 7 1341


Below will display a summery of disk space usage. You can find information on your mounts in the /etc/mtab file.
You might modify this file yourself from time to time or when mounting a device. If you insert a CD into your cd drive and your system is set to auto mount, below rapport will include this as a ISO9660 type aswell.

# df -h
Filesystem Size Used Avail Use% Mounted on
/dev/hda1 6.5G 4.7G 1.5G 76% /
tmpfs 237M 0 237M 0% /lib/init/rw
udev 10M 752K 9.3M 8% /dev
tmpfs 237M 0 237M 0% /dev/shm
/dev/hda6 29G 19G 9.1G 68% /home

Another way of verifying the usage of a HDU, partition or directory tree is to use below command.
I cut off much of the output because it will display a lot. You might want to filter or sort the output using grep or the like.
Check out the man page for this one aswell. Using this command is a handy way of finding large files if you are running low on diskspace. below is often used in shell scripts.

# du -ch
...
176M ./.cache/google-chrome
176M ./.cache
8.0K ./.putty/sessions
12K ./.putty
1.4M ./Downloads
8.0K ./.icons
12K ./.licq/history
164K ./.licq/users
4.0K ./.licq/qt-gui
224K ./.licq
4.0K ./GNUstep/Library
8.0K ./GNUstep
4.0K ./.opensync-0.22/engines
8.0K ./.opensync-0.22
36K ./Vocabularies
396K ./.gstreamer-0.10
699M .
699M total