Guide for Selecting a Power Supply

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Mod may want to make this a STICKY.
Here's a general guide on how to select a PSU for your HP desktop.  The majority of standard ATX powersupply (PSU) should fit inside an HP case.  The exception would be a slim line case or the esoteric Blackbird. 
Look for an 80+ certification on the PSU to save electricity.  A PC will normally idle or run at low load 75% of the time.  Therefore, look for a PSU that's more efficient in the 50-90W range.  An 80+ 350W PSU reaches the 80% efficiency level at 70W output (20% of rated output).  The output must be greater than 140W to achieve the same level of efficiency from an equivalent 80+ 700W PSU.  So picking a PSU that is rated 25% higher than the maximum ACTUAL load of the rig will save you more $ on electricity. 
A modern PSU is designed to achieve peak efficiency between 20 and 80% of the rated output.  Running a PSU above the 80% load level will result in excessive heat, noise, and premature failure.
An Nvidia rig with GeForce GTX 280 (1024MB on board RAM) will draw about 350W max from the wall.  If we factor in the efficiency of a 80+ PSU, then the actual load on the PSU is only 280W (350 x 0.80).  To calculate the required PSU, multiply the actual load by 1.25 (25% margin).  In this case we only need a 350W PSU (280 x 1.25) to run this GPU.
Let's apply what we know to a Radeon HD 4870 (512MB on board RAM) rig.  290W max from the wall.  Actual load is 290W x 0.80 or 230W.  Now apply the 25% margin rule and we arrive at a PSU requirement of 290W (230W x 1.25).  Even if we apply a 50% safety margin, the power requirement is still under 350W (230W x 1.50).
There is no need to overspec the PSU by 50% unless you run your PC at full load 24/7.  Keep in mind that there is a huge variation in PSU quality and rating.  A cheap 500W PSU may not be able to deliver as much power as a quality 350W.  Antec Earthwatts/NeoPower/TruePower, Corsair, and Seasonic are quality units widely available at many US retailers.
Many high-end PSUs will need additional power from the PSU in the form of one or two 6/8 pin GPU power plug.  You can purchase an adaptor to convert a 4-pin molex to 6-pin GPU power plug.
http://www.tomshardware.com/reviews/geforce-radeon-power,2122-3.html
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Found a review of the Corsair 400W (Seasonic-built power supply) that I've been recommending on this forum.  This ATX power supply should fit inside most HP case (minus Slimline).  It has ample power to drive a modern quad core plus Radeon 4870 or GTX280 video card. 
At 100% load, we see a 2.5% drop in the +12VDC rail, which is still within spec and does not affect the performance of any component inside the PC.  +12VDC regulation at 82% load is a superb 0.5% with 45C case temperature.  Note that the power output and efficiency decrease with higher temperature. 
Cheap power supplies are rated at 25C.  When these units are subjected to higher case temperature, the output can drop 10-20%.
Again, we discover the sweet spot of a good PSU is 20-80%.  A good 400W PSU is more than adequate for many PC users.  This Corsair is only $30 after a $10 rebate.  That's a great price for a quality/quiet PSU that won't put a huge dent in your wallet.
http://enthusiast.hardocp.com/article.html?art=MTYwOSw1LCxoZW50aHVzaWFzdA==
http://www.newegg.com/Product/Product.aspx?Item=N82E16817139008&Tpk=corsair%20400w
Additional 20% off w/ promo code "Mar20", ends 3/31
Message Edited by RoyalSerpent on 03-08-2009 11:40 AM
Message Edited by RoyalSerpent on 03-08-2009 11:42 AM
Message Edited by RoyalSerpent on 03-08-2009 11:45 AM