Does it matter if you have too much power? My old supply broke, it was a 350W ATX. what if i buy a 400W ATX (finding a 350W ATX is hard), will that cause problems?
Right now I have a 1.2Ghz Athlon Thunderbird with an Ali motherboard. I may get a 2.4Ghz Celeron or a 2600 XP with their respective motherboards if i need a new motherboard/CPU combo. Do those require a different power supply than the 1.2Ghz Athlon or is it all going to work with the same 400W ATX power supply?
ATX describes the type of power/connector the power supply has. It must match the connector/requirements of the motherboard and must fit into the the “power supply” spot in your case.
ATX has been the standard for a long, long time in computer years, so pretty much every full sized case with a modern motherboard you’ll run across takes an ATX power supply. Processor doesn’t really matter here, although faster processors may take a nominally larger amount of energy than slower ones. Most of your power is used to spin drives.
You cannot have “too many” watts, only too few.
-lv
Of course, having a power supply producing more watts than you need does waste a bit of electricity.
But that would be on the order of a penny or so a day – not very significant. Easily outweighed by the savings if you ever add options to your system, and need the additional power.
Not really. The load is only going to draw what it needs. There’s no “excess power” production. The only conceivable loses might arise from having a heavily overrated PS, so that it’s operationg far below it’s maxiumum rated load. in this case, the supply may not operate as efficiently as it would closer to rated max. This is much more of a problem with linear regulated power supplies than it is with switched-mode regulated regulated supplies, which are what is found in all computer systems. I don’t have figures handy, but I can’t imagine a switcher being more than a few percent less efficient at low loads than it is nearer to its design load.
Not necessarily, t-bonham. The components you have in the box draw a certain amount of current. If you have 2 HDs instead of one, your power supply will draw more power from the wall. If you have your monitor plugged into the power supply and turn it off, you’ll be using less power. The wattage of a PS is it’s rated maximum, not a constant.
Having extra power supply capability is also good for heat. If you buy a 500W PS but are only using 250W, it’s likely that less heat will be generated than if you had a 300W PS. Higher rated PSs usually have better components, like capacitors.
Bottom line is, go ahead and get a 400W. Check the internet for PS reviews. You should always check the internet before buying any new component…but I guess that’s what you’re doing now! 
[on preview, I see QED beat me a little, I’m posting cause I’ve got some extra…]
Actually you can. PC power supplies use switching regulators. Switching regulators are much more efficient than linear regulators, but they don’t regulate well unless they have a certain minimum load, which is typically about ten to twenty percent of their rated maximum. Some power supplies apparently have an internal load that they will switch on if you don’t draw the minimum required current, but some will either shut down or just won’t run properly.
For PC supplies though, you aren’t going to find one commercially available that’s big enough to cause you problems. Maybe if you get one of the largest power supplies available, and then got one of those really low power server motherboards. Other than that, I don’t think it’s possible to screw it up.
The power rating is the maximum. You can think of a switching regulator as something that turns the power on and off really fast, so that the average is just what the computer needs. The less power the computer needs, the more the regulator keeps the power “off” and therefore uses less overall power. In other words, if your comuter only draws 200 watts, the power supply only produces 200 watts, even if it is rated for 350 or 400. (actually if you want to nit-pick, it would probably produce about 210 watts, since it’s not 100 percent efficient)
I bigger power supply might also be louder than a smaller one (if it has a fixed speed fan). And more expensive. 400W replacing a 350W is OK, but no need to get a 1500W PS.
Brian