Okay> I think we've thrashed this subject to death...
...but I just thought having read through the posts that I would add my 6 pence worth.
As a hardware engineer and looking at this from a large corporation server point of view.
The mainboard can take upwards of 30w. 30w being about the minimum of a basic ATX form factor (ff). This will rise on the later ones, although the smaller ff boards have been designed with power consumption reduction in mind. Also mainboard that include a mezzanine layer or riser boards will take more.
This does not count the RAM or CPU. Ram is rated at 10w per 128MB and that is the calculation I use for Xeon Dual & Multi-processor server machines using 100 or 133MHz SDRAM DIMM modules and, of course goes up with frequency, although voltage comes down a little. A PC3200 runs at 400MHz DDR & 2.7v, with this and other design enhancments the calculation would still be about the same.
CPU's are quite power hungry. Intel have got some better models out for the laptop, like the Centrino and Mendocino, but they are normally classed, for a single component, as the largest uses of power, with the Intel P4 normally using more power than the AMD Athlon. Compare a 3.4GHz Intel 90nm P4 and an AMD Athlon 90nm 64bit 3.5GHz and the power consumption is about double in the Intel chip. I would say to allow up to 100w (though see below), with the newest P4 600 series, even though they have 125million transistors and double the ATC L2 cache to 2MB, actually using less power. The hyperthreading technology in the Intel CPU uses about 10% more power but for shorter bursts as it finishes tasks quicker.
The calculation is (Total Capacitance*frequency*(voltage²))/2, hence a 3.2GHz Intel P4 'C' CPU using 1.5volts internally will consume ≈ 70w whereas the Intel P4 mobile 715 running at 2GHz consumes only ≈ 16w. The 64bit AMD Athlon FX-55 running at 1.5v internally consumes ≈104w and the latest Pentium mobile CPU core code named 'dothan', this is in the 'sonoma' CPU with a 90nm process, 2MB of L2 cache and a speed of 2.13GHz (Pentium M770) uses only about 27w. There is another techonlogy that I have not investigated, but this is called 'Transmeta Crusoe' and is based on allowing the ouyter layer functions of a CPU to be carried out in software. The CPU consumes a lot less power and is well suited for smalled portable devices. a 700MHz with a core voltage of 1.6v uses only 3w. It is x86 compatible.
So a basic 30w consumption mainboard with 1GB RAM and a 3GHz CPU could be using 175w
ADD;
5w for the floppy (if you still have one)
25w for a 50x CD-ROM or 10x DVD-ROM
30w for a 4x AGP card (beware here some cards can consume vast amounts of power. An ATI Radeon 9700 Pro GPU uses 45w. Some have 2 molex power sockets on them, so use more power. Those normally recommend at least a 350watt power supply.
PCI cards (33.2MHz x 32bit) use about 5w each
IDE 5400 drive = 10w IDE 7200rpm = 15w, but all harddrives use a lot more power when they are spining up and this should be allowed for. A 7200rpm will use up to 30w and a 10,000rpm up to 40w. Having said that the newer models of drive are better at managing power useage and may be slightly less, but err on the side of caution.
SCSI harddrives use more power 7200=25w, 10,000=40w and more during spinup.
It is always sensible to go for a good model. If you buy a decent case you should get a decent PSU, but you can always do better and as has been said a few times, branded models are going to be better. It makes sense logically. The company is proud of there equipment and so brands it.
I would go for a minimum of 350w today and normally 400w
And better models are quieter, have a cleaner power output, are less likely to have problems with the Rectifier. If that goes wrong there is no telling what damage you could do to your computer as AC power could get through to your components.
Have fun.
