Is my PSu enough?

then why my games lag???
i have
motherboard -gigabyte g 31
processor-intel core 2due 2.8 E7400
ram - 4gb DDR2
GPU-Sapphie readon hd 7730
my psu was with casing it has 450 wat
my all games lags..
when i start the game there is 55-60 fps
but when i start a mission it falls to 15-25
it happens every time
 



then why my games lag???
i have
motherboard -gigabyte g 31
processor-intel core 2due 2.8 E7400
ram - 4gb DDR2
GPU-Sapphie readon hd 7730
my psu was with casing it has 450 wat
my all games lags..
when i start the game there is 55-60 fps
but when i start a mission it falls to 15-25
it happens every time
 
Your games lag because your processor is pretty weak by today's standards. A Core 2 Duo is quite an old chip. You will need a more powerful processor.

By the way, it is funny how you ask whether a "normal" PSU is sufficient without giving any details on your particular one.