Your system supports 4GB DDR3 - 1333MHz memory and you do not have to change the motherboard as suggested in Post #2 .
Your motherboard supports two memory types namely DDR2 and DDR3. Both DDR3 and DDR2 memory cannot be used simultaneously and hence you need to choose the memory you wanna go with.
First thing, you ought to do is determine if the existing memory is DDR2 or DDR3. There are many ways to determine it. If your system is connected to the internet then please do a scan using the following link - http://www.memorystock.com/memscanner/SysInfo.exe
If your system is not connected to the internet, then you can check it physically. There two ways you can do that.
See if the memory is installed in black or the blue slot. Black slots are ordained for DDR3 memory and blue for DDR2. This way you can say what you might have.
Remove the installed memory. You can find the specifications mentioned on the memory label. If it says something like PC2-**** then it is DDR2, if it starts with PC3-**** then it is DDR3.
This way you can determine what your current memory is. Then based on the installed memory you can install the 4GB module. Note: As mentioned earlier you ought to remove the DDR2 memory to install DDR3 memory.
P.S - I'm a proud owner of the same Mobo and I use DDR3 and it works perfectly, so you can take my word on it .