Buying a motherboard?

I wanted to upgrade my basement desktop. It is a HP M9402F. The only difference is that I added another CD/DVD drive (Hp which cost 49.99 at best buy). And a graphics i think NVIDIA 9400 GS or GT cannot remember. I wanted to upgrade my CPU to a much higher level. I wanted to get the AMD 945 but the Candra R store in NYC told me they hardly make those anymore. So I was forced to get the AMD 965 3.4 GHZ black edition. It fits perfectly but I think it overpowered my computer. I brought a 450 watt power supply. Antec vp450 120mm fan black box. Cost my 40-50 plus tax. I wanted to upgrade for gaming purposes but still be affordable. My computer shuts down when i turn it on. Do I need to upgrade my entire motherboard? What can all of you tell me?