at 5v, they'll make little noise and move about 20cfm
More like 10 cfm. They are virtually silent at 5V -- if there is no impedance on either side of the fan. Impedance is anything that blocks the airflow, usually a distance of even 1/2" is audibly quieter. This is one reason I recommended the Zalman-style fan mount -- sort of hanging over the HS but not touching it. Ditto cutting away grills from air vents -- on a LianLi, which is so pretty, this would be going against the grain. Also if the airflow is real low (as with 5V) the difference is pretty small.
Noise is additive, somewhat. Two identical 10 dBA fans will make 13 dBA. 3 will make something like 15 dBA. Whether 3 very quiet fans or 1 quiet fan with the same CFM is noisier is really difficult to predict, just have to try it. Main advantage of multiple fans is spot cooling, which still remains extremely useful & usually necessary.
I do have a P3-550 oc'd to 733 and undervolted to 1.45Vcore that I run more or less fanless. There is a 4.5V Panaflo in the elcheapo PSU, a massive GlobalWin VOS32 HS w/o fan, and a 4.5V Panaflo on the back panel, blowing out, very close to the CPU. It's my most stable machine, never crashes despite gazillions of software on it. The CPU generates only 15W heat at this setting.
Whether the P3-1GHz can be run fanless with the SLK800: Stock, it puts out 26-34W depending on which variant you have. For details, look for the link to Processor Electrical Specs under Links, in information sites. Also, go the the Links area, look under software utilities for Radiate http://www.silentpcreview.com/modules.p ... file=index
This is a great little software that let's you calculate temp of CPUs based on HS c/w, Vcore, core speed, and CPU type. Lacking the most recent CPUs, but still very useful & fun to play with, extremely educational.
Looks to me if you can undervolt stably to 1.5V or less, you CPU power drops to ~22W and you MIGHT have a chance to cool it passively -- but again, I don't know what you gain against a 5/7V Panaflo.