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From what I've seen, the most powerful mATX PSUs are the Enermax EGP285 and Fortron's mATX offerings (don't remember model/wattage, but it's less than 300w overall). They're quite powerful on the all-important +12v rail for their size, however AFAIK shuttles do not use a mATX supply, instead more of a 1u sized supply, which you can get quite powerful varities of for rackmount servers. From what i've seen from a couple of my friend's shuttles that are running GTs, the most powerful one that shuttle themselves offer is enough, with a little of extra cooling, to power a GT, but not an ultra. As in, it'll boot and run fine with the GT but not the Ultra. YMMV, but I hope this helps