|Make sure you do not have metal mounting posts under the mboard where there are no screw holes in the mboard.|
Make sure there are no loose screws under the mboard.
The cpu fan must be connected to the cpu fan header.
See response 2 in this - try cleaning the contacts on the ram modules, and making sure the modules are properly seated:
If you have a speaker or other sound device connected to the proper pins on the mboard.....
The mboard should beep one short beep while booting if nothing's wrong
If you have installed ram that is not compatible with being installed in the mboard, or if there's something wrong with the ram, you may hear no beeps at all that the mboard will not boot all the way.
If that's your problem, if nothing else is wrong, if you remove all the ram and attempt to boot, you will get no video, but you should hear a different beep or a pattern of beeps that indicate no ram installed.
Apparently, most sources says a HD 5450 video chipset on a card requires that your system have a minimum 400 watt PS, however it should also be able to supply at least 22 amps current at +12v, total, and some older or el-cheapo 450 watt PSs may not be adequate regarding the amperage.