yes, typically you can mix and match ram sizes in a motherboard. We would have to know your exact mobo to know what capacities/voltages/etc. your mobo will take, but generally speaking, if you match the specs of your current stick of ram with what you purchase new then everything should work fine.
If you get ram with slightly different settings, just know that all memory will run with the slowest speed and timings between both sticks. So if you have 1 stick that is DDR2 800, and annother stick that is DDR2 667 then both sticks will run at the slower 667 speed.
Keep in mind that running mixed memory does not allow the use of Duel DDR, which can give a nice speed boost if your motherboard supports it, so I would generally suggest that you only use ram in identical pairs for best performance. But if it is an older system, or performance is not a large concern, then don't worry about it.
Lastly, and this is very important, you cannot mix ram of different generations. If your system uses ram that is DDR2 then it will not work with DDR1 or DDR3.
If you have specialty ram, meaning it uses lower than normal voltage, or is buffered, or uses ECC error correction, then you will have to match the type of memory used exactly in order for the computer to work. These types of memory are very rare, but it is something to look out for (especially the voltage).