Yep, it really depends. Low end cards that are newer use less, because they have less going on, and are a smaller nanometer build process than older low end cards. Similarly, high end cards use a lot of power under load. With dual GPU cards, power consumption is almost double what a single card would take. For instance a 4870X2 takes almost double the power a 4870 does.
I haven't used that Micronics 550 psu so don't know what it can do on 12v. A generic 550w psu usually has less 300w on 12v. That's something needed to be considered, Also, It is P4 so don't pick any GPU over 8800GS.
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