How big of a Power Supply do I need?

a standard 750w should be good for now and if you add a bigger gpu latter on. the newer gpu from amd and nvidia are now using less power then there older lines. if you want to sli or crossfire your rig then you may need an 850w or larger unit. it the amps and watts on the 12v line. on a gaming pc make sure your power supply has one larger rail and not a split rail unit.
 

avarice

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May 10, 2006
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Your current build will work fine with a QUALITY 500-550W PSU. If you want some head room - get a 550-650W QUALITY PSU and you will be fine for a single card upgrade (for now.) If you wish to do SLI - you may want to go 650W to 750W QUALITY PSU depending on the card.

Have I mentioned the need for a QUALITY PSU - Buy Seasonic, zippy, or Superflower manufactured PSUs (not neccesarily branded by these companies - as XFX is built by SeaSonic.)