Yo. I need an four amps. One combo amp, one amp head, and two amp cabs. Iv'e got the amp head and the combo amp, but i was looking for which cabs to buy. I want the loudest possible for the flexibility of playing by myself, practicing with my band, or at a gig. Iv'e come across an incredible 600 watt cab, but i don't know if thats neccessary. And i don't mean in price. Price is not an issue here, But is a 600 watt amp GARUNTEED to be more powerfull than a 400 or a 200? A yes or no answer will suffice.

NOTE: The amp in question is the Krank Krankenstein Dimebag Cab.
The wattage on a cab only refers to the amount of power it can handle, not how much power it actually uses. A 30W head through a 600W cab only has 30W of output.
I'm really not sure of 1/2 of what you said, but you realize a cab and head are 2 different things right? A 600 watt cab doesn't really mean anything. It just means it will take a 600 watt load. Example 100 watt cab + 75 watt head = Good. A 200 watt head + 100 watt cab = bad. The power is in the head. Good to match the cab to that power. The difference in the volumes will be with speaker size and how much air those speakers can move.
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The Krankenstein is a 100W head so any decent cab should do example avatar, vader, marshall or krank.. And 100W head is more then enough power for any gig you will have..

But im still a little confused on what you actually asking?
The wattage rating of a cab doesn't mean more power, just how much it can handle.
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For a cab, more watts=more power handling.

And you only have 2 amps, a head and a combo are both amps, cabs are not amps, they are speakers.

Why exactly do you need both a combo and a stack? Plan on playing any stadiums any time soon?
like mentioned, cab power doesn't matter, it just tells you the maximum you can put into it without things burning

but there are other things
better speaker efficiency = more volume
some speakers are just louder than others if you put them on the same amp

for example, from the celestion site:
the greenback "Sensitivity 98dB"
the vintage 30 "Sensitivity 100dB"

thats 2dB difference which is actually a whole assload if you'd want to up your amps max power