It will down clock the cards to the slowest of the 2 and will only use 256mbs of the 512mb one. And for stability always put the slowest card in the top main slot.
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NZXT Apollo
Phenom 8750BE 3.1ghz.
Gigabyte GA MA770 DS3
G Skill DDR2 1066 4gb.
PNY 8800GT/VD964
W/D RE2 Sata 3.0 500gb.
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