I have just recently purchased a gigabyte GA-Z77-DS3H motherboard, core i5 3570k processor, AMD Radeon HD 7800 Series and monitor etc. I originally purchased 8gb of corsair vengeance 1600mhz ram (2x4gb) sticks which has taken up 2 expansion slots. the build is working perfectly and I am very pleased with it however, I just recently purchased another set of corsair vengeance 8gb (2x4gb) sticks and when filling the vacant ram spaces of which there are two available my computer will no longer boot up. The memory is exactly the same (CMZ8GX3M2A1600C9). I am using a 750 watt corsair PSU so i really dont think it has anything to do with the power usage.
could anyone please point me in the right direction as how to fix this.
Remove the recently installed RAM modules, and then try again. If you're unsuccessful, test the PSU with a digital multimeter. It is a stretch, but certainly not impossible that your PSU crapped out. If you don't have a DMM, or can't/don't want to get one, try breadboarding to rule out shorts. Breadboarding, and other common areas to check can be found in our troubleshooting guide. Click on the link in my signature.