for a moment there I thought you were planning on direct die cooling each individual ram chip
but assuming that you're confident that you can mantain a water tight seal around the ram sinks it should be possible.
[NOTE] if you fail to seal properly around the edges of the ram and water get's underneath the BGA chip (which it can) then it'll corrode the solder balls and
will kill the card.
ok the bga chips on the 9700's are 11mm square, measured with a steel ruler by eye