New System build trouble

I am not new to the world of computers but when it comes to completely building a new computer I am definitely not seasoned in that area. Recently I've decided to build a new computer. With a Cooler Master Elite 310 case, AMD Phenom X3 8750 2.4 Ghz CPU, 4GB (2X2GB) Crucial Rendition PC2-8500 DDR2, XION 630W Simple Power Supply, and ECS Black Series GF8200A. These were the new parts I ordered and the rest (HD, Monitor, etc.) I already have. So I follow the exact order the motherboard manual says to set everything up and it doesnt power on. I try unplugging and plugging everything back in carefully to see if i screwed up the first time and still the comp wont power up. I test the power supply and it works fine. I take the MoBo out and do a POST test with just the CPU, PSU, RAM with the monitor and keyboard connected and I can access the BIOS just fine. But when I go back and put the mobo back in the case and connect all the hardware nothing works. My guess is the case is causing a short somewhere, but how do I fix that? Any ideas?