The PSU is unlikely to have anything to do with that unless you have upgraded the components and are starving it of power, in which case it would likely just die on you.
Assuming you haven't change the original specification, you have a Pentium D930, 1GB DDR2 (533MHz) and a Radeon X300 SE with 128MB dedicated RAM.
That is likely your reason for slow down, the games will be stuttering, slowing down and freezing because the intensiveness of the games are overwhelming your processor, RAM and graphics card.
Changing the power supply will make no difference to that I'm afraid to say.