RAM can be added and taken out easily from the computer system
easily.
However there are some points to note when buying new RAMs:
1. As most motherboard only supports one type of RAMs, you need
to know the type of RAM you are currently using.
It is recommended to buy the same speed and ratings as the one you
are currently using, unless you are sure your motherboard can
support rams with faster ratings.
Eg. You are using DDR2 with 800mhz speed, you should try to get a
DDR2 with 800 mhz speed also, unless you are sure that you
motherboard can support DDR2 with 800mhz or faster speeds.
2. The amount of empty slots you motherboard have, if you only
have 1 more empty slot, dun buy 2 pieces of rams 
After buying rams, be sure to install rams securely, a lose ram
can cause the computer not to boot up.
In an event that the computer doesn't boot even if you are sure
that it has been securely installed, try nudging the ram left and
right.
It is a good practice to clean the ram slots before installing
new rams to keep it as clean as possible
anyway to tell the speed of my present RAM without opening up
the PC ?
also anyway to tell if i can add 1 more RAM without opening the
PC ?
my current PC using 1GB RAM so if i add 2GB, is that ok ?