You can't stick a DDR2 module into a DDR socket. A DDR socket is a 184 pin, as opposed to the 240 pins on DDR2.
You need to check in your MOBO documentation and see what kind of DDR sockets it has. You might even have the DDR2 240pin sockets already - DDR2 comes in ranges of PC2-3200 ( 400Mhz ) to PC2-6400 ( 800Mhz ), so even if your computer has the PC 3200, it might be the DDR2 type.
However, if your sockets are the DDR 184pin, your max speed for memory is 400Mhz, which is the max for DDR! If you have the 184pin type, and try to buy the PC4000, it will simply wont fit.
Now, if you are not sure about the type you have, or can't find the documentation, just pull one module out, and google for 'DDR DIAGRAM' or DDR2 DIAGRAM'. Google will pull up more than enough pics for you to look at and compare the connectors on your memory to the one on screen.
Hope this helps.
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