It is that simple providing the new RAM stick is compatible.
Check your motherboard manual to find out :
- whether your system takes DDR or PC133 for example.
- The 2 sticks don't have to match in size but check the maximum RAM size your mobo can support
The existing stick should have the FSB printed on it (i.e. the base frequency) or a number that indicates the FSB. Get a new RAM stick with the same FSB, & if possible matching CAS latency.
If, by any chance, your motherboard is dual channel however, the 2 sticks have to be absolutely identical.