As far as Im aware, if your pc uses SDRAM memory then no you cant use DDR memory. If you have any free slots then by adding more SDRAM from Crucial could speed it up a bit. This would be easier and cheaper than upgrading your processor.
To upgrade the processor you will have to find out what type of socket or slot the processor fits into for compatibility with a faster one.
Another simpler idea could be to remove some unused programs from your computer. If there are lots of programs running in the background these will all take up resources and could slow you down.