Take a look at your motherboard, Tweakboy.
I mean seriously, open the case, remove your memory modules, and take a look at the actual physical memory slots.
DDR2 and DDR3 are physically different. The little groove on the sticks that match up with the tab in the slots are in different locations. They can only fit in their respective sockets.
Are all your sockets on your motherboard identical?
Try it now.
See if you can actually physically fit one of your memory modules into each of the sockets.
I'm waiting...
Do they fit?
If the answer is YES, go to answer 1.
If the answer is NO, go to answer 2.
Answer 1. --- Your motherboard only supports DDR2. You can not actually physically fit DDR3 into the slots.
Answer 2. --- Your motherboard is a type of hybrid board that supports both DDR2, and DDR3 (but not both at the same time). The slots into which you are not able to fit properly your DDR2 modules are designed for DDR3.
As recommended previously, save your money for a new setup, since new memory will not make a difference that you will be able to notice. GG, GB ,FF, FO, @ GD