I’m afraid it’s not possible, I have a quite old computer and my mainboard (Intel D915PGN) won’t work with DDR3. This is what is written in the instruction in summary:

Memory: • Four DDR SDRAM Dual Inline Memory Module (DIMM) sockets • Support for DDR 400 MHz and DDR 333 MHz DIMMs • Support for up to 4 GB of system memory

I hope I interpreted it in the right way, I mean, DDR means DDR and not DDR2 or DDR3, right?