You'll have to take into account that PSU with it's very unevenly balanced 12v rails and 420W MAX 12v wattage rating too ( 35A @ 12v ) ..
420W @ 12v on a balanced 12v rail PSU = 17.5A per rail, who knows how they've managed to rate it @ 12v1 - 25A / 12v2 - 18A .. Unless you know how the rails are balanced when under load, then it's going to be tricky suggesting something really ..
Maybe a HD6xxx series which doesn't require more than x1 PCI_e 6pin connector .. I think you'd be pushing it too far with any other card which uses x2 6pin power leads ..
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