Power Supply Ratings Just Get It... I'm Trying To Figure Out What Video Card To Get And If I'll Need To Upgrade The Power Supply?

I'm trying to figure out what video card to get and if I'll need to upgrade the power supply? - power supply ratings just get it...

I have a new HP a6152n with an output of 300 watts. I want to upgrade the video card, so you can have two monitors .. I searched Google for video cards recommended for my system and everything that the players continue to find updated maps are ridiculously powerful video and find that they need their power as a point in the video card upgrade more energy.

All you need is a good PCI-Express card under Windows Vista with Aero enabled, no problems dual monitor 22''.

Can I have a 128 MB PCI-E card back with good marks in the egg? If a PCI-E 128MB enough for Vista and two monitors from 22''? I will also upgrade the power supply? If so, what should the watt of the new diet?