parallel vs series,
All cards are not created equal,,, meaning that two cards on air, same manufacturer, same fans, same builder and application methods, one card will always run hotter.
To run parallel on a single pump both sides must be even or balanced in the plumbing department which is almost impossible to achieve. One side will always be a little weaker flow that will over time accumulate the sediments and become weaker still causing your temps to vary that much more. Two pumps running parallel would alieviate this issue a bit but still you would have two different temps because now you two cards and pumps and sets of plumbing,,,,
Did you did mention something about cost in one of your posts somewhere? Empty your piggie bank and get out your credit cards, this ain't gonna be cheap.
Did I mention balanced plumbing? All tubing lengths on both sides must be equal, all bends in the tubing must be equal radius, when you install a wye you need to use straight pipe to balance the flow otherwise it will favor the outside of a radius, complicated stuff to do in a case you want to build neat or clean and you have limited space.
In a series loop the imbalance can work in your favor. Test the cards on air first, you'll find one card hotter or one slot, maybe from voltage or airflow in the case, any number of possibilities, but find out first on air then run your loop accordingly. As Grooveriding mentioned loop order is not that critical as you may think. The water temp from any given point in the loop does not vary a measurable amount, it is simply moving too fast but like him I too have always tried to build my loops in an order that satisfied my mind as "proper". Pump to the rad and then to the blocks to provide the coolest water possible. right, wrong or indifferent, that's what makes me happy.
Get enough rad to cool your loop w/o excessive fan noise and you will have built a good loop, other than that there is nothing to it!