As far as I know it`ll have to be another 9800GTX+ memory and clockspeeds do n`t matter, SLI will slow the faster card to match the slower and a 1Gb card will have 512Mb of its memory ignored if it`s paired with a 512 Mb card.
Some may question the wisdom of pairing two such relatively weak cards with a high end CPU like the i7.
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