Let's start with something simple.

To convert Amps to Watts:

Watts = Amps x Volts

This is why--when assessing which PSU to get-- it's important to look at the 3.3v, 5.5v, and 12v "Amps". The 12v rail is arguably the most important since your CPU, Video card, hard drives, optical drives, etc, draw power from it.

Here are some max-power draws expressed as Watts:

**AMD X2 4200+: 89 watts MAX**

Dual 7800GT in SLI: 112 watts MAX

Two Raptor 74GB in RAID 0: 16 watts max

Extra Seagate 7200.9: 12 watts max

TOTAL: 229 watts

Dual 7800GT in SLI: 112 watts MAX

Two Raptor 74GB in RAID 0: 16 watts max

Extra Seagate 7200.9: 12 watts max

TOTAL: 229 watts

Obviously, this doesn't include some other minor things, but it is more than ample to give you an idea of the actual wattages involved here. Also, let's keep in mind this is about maximum PEAK LOAD on every component. This system would draw significantly less during normal non-gaming use.

Based on the formula I stated above, that means an AMD X2 4200+ expressed as "MAX AMPS REQUIRED" on the 12v rail would be:

**AMD X2 IN AMPS|**

89 watts / 12 volts = 7.41 amps

Single Geforce 7800GT IN AMPS|

56 watts / 12 volts = 4.66 amps

89 watts / 12 volts = 7.41 amps

Single Geforce 7800GT IN AMPS|

56 watts / 12 volts = 4.66 amps

You can factor in 2 amps for the three hard drives. 1 amp for cooling. These are 12v rail items as well.

This puts our test system at ~20 amps (rounding up).

Now let's go ahead and assume 10% overhead. Just to build in a ton of extra breating room. That puts us at ~22 Amps or ~260 Watts for our main hardware. Need another hard drive? Add an amp or 10 watts. Going to add another CPU? 90 watts or 8 AMPS.

So, let's back track a bit and think about this. If our test system has nice overhead @ 22 amps, with 3 hard drives, an AMD X2 and 7800GT SLI setup.... what PSU can we get?

http://www.newegg.com/Product/Product.a ... 6817104154

OCZ Powerstream 450 watt: 12v RAIL? 26 AMPS (that's enough)

Cost? $90

http://www.newegg.com/Product/Product.a ... 6817192002

Atrix 500 watt: 12v RAIL? 15 AMPS (no way... nowhere near enough).

Cost? $29.99

**So, yeah, Atrix has a "500 watt" power supply, and it's cheap... but it's completely insufficient for our system. There is no way the configuration would run properly on that "500 watt" PSU. The "450 watt" OCZ powerstream, however--SOMEWHAT against conventional wisdom that you need 500 watt or 600 watt PSU to do SLI--would readily run our test configuration.**

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Here are some other common wattages:

Radeon X850XT = 45 watts @ idle / 68 watts @ peak load

Geforce 7800GT = 39 watts @ idle / 56 watts @ peak load

Radeon X1900XTX = 49 watts @ idle / 120 watts @ peak load

Geforce 7800GTX = 53 watts @ idle / 94 watts @ peak load.

And AMD X2 Manchester (3800+, 4200+) run @ MAX 90 watts. The Toledo cores (4800+, 4400+) run @ MAX 110 watts.

A Raptor 74GB draws 8 watts.

A Seagate 7200.9 draws 12 watts.