Try going back to your original RAM configuration. If it boots then you need to make sure that the RAM you are putting in is a good match for the RAM you have in there. Be aware that WIN 7 is fussy around RAM. If you are using WIN 7 try changing the RAM brand.
You can also look up your motherboards name and specifications. This will help you figure out what will work in your system - moreover if you go to the manufacturers website, they might have a memory compatibility list for you to choose from.