what do you mean by "horizontally"?
The water will flow at the same speed, regardless of mounting direction. More likely you'll be limited by the mounting holes in the mobo.
I ended up mounting mine diagonally:
Hi rusty, that looks very nice using the spare gpu cooler for the north bridge, just wish i had seen that picture 2 days ago because i just installed my reserator 2 days ago for the cpu (mobile 2500 @ 12x200 1.65v) and the gpu only (crucial 9800pro).
however I decided not to use the pci slot as the way out for the water tubing because it seemed to me as if it would be very hard to get the graphics card out (my next upgrade) as there is very litte movement if any, so i used the the 12cm fan hole at back of case for the in and out water tubing which make it very easy for me to change/upgrade parts.
I do have a couple of questions though..
1) does anyone leave there reserator on 24/7 even if their computer if off?
2) what to people have the reserator sitting on, as i have it on my desk just now and heard the pump vibration though the desk which was annoyng, so have now got a ceramic wall tile wraped in a rubber non slip mat which helped big time but not 100%.
3) would the following set up work: silverstone lascala lco1b case, antec phantom psu with my current gear - 2 optical drives, 80g samsung spinpoint h/d, abit nf7-s v2.0 with mobile 2500 @ 12x200 @ 1.65v & crucial 9800 pro both watercooled with the reserator.
p.s the only fan in my system at this moment is the one in the nexus 3000 psu, which doesnt seem that quiet once every thing else is silenced.