[acidrain]

I have not had to replace cartidges yet (we have pretty clean city water in Seattle... less than ~25 PPM), but like Luminous said, they all are the same dimensions, and should be interchangable.

Rad, I feed my 50gal storage tank with a float shut-off valve, and had to fiddle with the bypass flow valve a bit to get maximum flow.
Sometimes I'll use every bit of my stored water on late night runs, and by the next day, the tank is completely filled.
You'll find that more bypass water flowing over the mebrane will yield maximum RO water. Also, line pressure makes a big difference. They sell small pumps that incease pressure, and recirculate water for less waste. If you have low pressure (less that 60psi), you might check into those.