You'll be lucky to find 4GB dimms capable of the speed you want.
But there are plenty of 2GB dimms capable of PC19200 (DDR3-2500hz) and PC3-20000 (DDR3-2600hz) straight out of the box.
Also, for simplicity, you could also consider ram that already includes a watercooled heat spreader, such as OCZ Flex PC3-19200
Then it is just a matter of buying enough sticks, slotting them in, connecting them to the chilled water loop and you're good to go
Whatever you do, I'm subscribed as this will be one hell of a build if you ever pull it off