Ram upgrade wont work

Hello, I am currently running a packard bell ixtreme with a pentium 4 2.4ghz , 512 mb ram, 256mb graphics card (not sure what type), with 7 ultimate. I recently purchased 2 512 mb ram sticks to upgrade my ram to 1gb, when I put in the ram the computer started making its usual noises however the screen did not start, after checking everything was connected properly I tried resetting the bios, and sucess it started up and registered the ram on the boot screen however when it got to the windows 7 boot screen it the went off again. I restarted the comp and it had went back to the screen not coming on at all! Ive checked and my mobo can take the ram upgrade according to the crucial site, also tried plugging the monitor directly into the mobo graphics port to no avail.. The only thing I can think is to reinstall an os. Does anyone have any ideas on what the problem is?