Define "all out." If you are only going to cool your cpu and a single gpu you wont need more than a single pump, anything above that and you will see minimal performance increases. However, if you are planning on adding more components to the loop then it may be advantageous to you to get a 2 pump setup (ie cpu, 3 gpus, 2+ rads.)
Also, adding 2 pumps in series will NOT...let me repeat that, NOT double your flow. Because most water cooling pumps are centrifugal pumps you need to look at their pump curves as well as know how flow is affected when you add pumps in series or parallel. If you add pumps in parallel you will double the maximum flow rate through the loop, if you add pumps in series you double the amount of pump head (differential pressure) across the pumps. You also need to know that the amount of losses your fluid will see when flowing through your loop is proportional to the total system resistance as well as the flow rate squared.
Long story short, if you have a high restriction loop, use 2 pumps in series, which is what the MCP35X2 is, which is also why it is so expensive. If you are going to use that type of setup I strongly recommend compression fittings (or hose clamps) because the increased pressure of the loop will make leakage more likely. Also keep in mind that the more flow and restriction you have in your system the more suction head (NPSH for you engineers) your pump will need so ALWAYS put your res above your pump, just to be sure, because cavitation is no ones friend.
If you are only going to have a smaller loop, you will be fine with a single MCP35X or a 655. I used to run a loop with a D5 (essentially a MCP655) and I never had any problems.
If you have any other questions please ask, I know a thing or two about high pressure systems.
Edited by lithgroth007 - 2/25/12 at 7:59am