I just wanted to share with you the specs of a small renderslave I just built (CPU only), trying to lower costs whilst keeping a decent performance. I thought it might be of interest to some of you. I intend to build a second one soon, with close specs, but different parts, trying to have it even smaller, I'll update this topic then.
A quick disclaimer first: all this is very subjective, and only reflects my own experience. It is not scientifically tested
And secondly, the brand names are quoted for accuracy, but absolutely not with the intention to praise for such or such brand nor to boast on my rig 
In order to explain a little of the background, I have a rather powerful laptop as my main computer (ASUS G73JH-TV008, that's a Core i7-720M with 8Gigs of RAM, Radeon HD8570M, and I chunked an SSD to replace the first system hard drive and make it faster). It used to be dual boot win7 64b / Ubuntu 10 64b but I found I was using more win7 so I removed ubuntu temporarily.
However, it was running quite hot and very loud when crunching on a render, and the processor, although able to launch 8 threads at the same time, was not that fast, topping at 1.6GHz, although it is supposed to boost at 2.8GHz) . So I thought I might build a small render slave to help.
For the moment, I decided to go for CPU only, so no hybrid rendering (i.e., I'll be using an integrated graphics card in the beginning). I went for AMD based system rather than Intel, as usually the components (mobo and proc) are a little cheaper.
So here are the specs:
- MoBo ASUS M4A88TD-M EVO/USB3 (SATA3)
Proc AMD Phenom™ II X6 1090T Black Edition
Corsair CWC-H60 (watercooling kit)
8Gigs of RAM G.Skill Kit Extreme3 2 x 4 Go PC10600 Ripjaws CAS 9
SSD OCZ Agility 3 - 60 Go (noiseless, low consumption, awesome speed)
Wifi Linksys WMP54G on PCI port
The case is a noname micro-ATX case with integrated 400W power.
No keyboard, no mouse (I use spare USB ones that I have), no screen (I used my main external screen just for the initial set up), no slim DVD drive or whatnot.
Once I got the SSD recognised (had to create a blank partition first), I booted a USB key with an ISO of latest Kubuntu 64b (11.10), just because it relies on QT, as is Luxrender GUI if I'm not mistaken. Everything was recognised smoothly by ubuntu (even the wifi adaptor). I set it so that it opens a session without asking for the login, faster, easier.
I then installed the latest lux dev from SATtva's weekly builds (had to add a few packages, but that was simple), then symlinked it to my home directory. Then added the following to my .profile:
./luxconsole -s -t 6 -q -W &
That way, as soon as my system boots up (which it does in less than 30s from power on, thanks to the SSD), it is ready to be controlled by luxrender GUI from my main machine.
The stock AMD fan was very noisy
. So I mounted the CWC-H60 and rebooted. And it was even noisier! 
So I installed xsensors, and found that the fan was running full speed. Heh
Hence I went on installing lm-sensors, ran sudo sensors-detect, then ran pwmconfig to set the fan thresholds I wanted (900rpm under 35°C, 1500 when reaching 50°C, and full speed when going above) and finally applied the settings with sudo sensors -s. And all of a sudden it was very very quiet (at least in idle mode).
So now when idle my system runs at less than 30°C for the CPU, and seemingly the same temp for the mobo, with watercooling fan at the minimum (around 915 rpm).
When launching a render, the 6 cores go 100% load, and temperature starts to raise. I let it run a few hours. It only reached 47°C (fan was around 1400, so I started to hear it over the general ambiant sound of my living room), knowing that the room temperature is currently 19°C-20°C.
What I could enhance now: on the hardware side, I find the mobo is actually running warmer than with the initial stock cpu fan, as the default way of mounting the CWC-H60 is pulling air in, so the air inside the box is heated by the CPU via the cooling radiator. It is usually between 32°C and 45°C, which is getting a little high for a mobo. So it could use some additional cooling 
add a chassis fan to blow fresh air in. Or maybe reverse the mounting of the CWC-H60 so that it pushes air out of the box, with maybe a second 12cm low rmp fan to complement the Corsair stock one. I could also try to overclock the processor (Black Edition and the mobo are meant for it), and see if it runs stable and cool enough when closing 4GHz.On the software side, as I do not intend to plug a screen, I should entirely disable the window manager, or maybe go to a lighter one than KDE, like XFCE. KDE is maybe eating a little power for nothing. Or even, a simple telnet will do
.I might install proprietary ATI drivers though, just to get OpenCL support for the CPU (dunno how to do it other way), and test with LuxMark with native CPU (the integrated HD4250 GPU does not support OpenCL).
Conclusion: so all in all, I find it quiet, powerful, not too expensive: total was roughly 500€. Please bear in mind that I took all the parts from a single vendor, and therefore probably did not get the cheapest possible. Moreover, it was bought in France, with a quite high VAT (19,6%). Residents from other countries looking for the same parts might be luckier with that.
Now you linux heads might have some ideas on better tweakings
, or even how to gently disable the window manager (I don't know yet). Please do comment, it's always good to have feedback to gain expertise.Thanks for reading this awfully long post
. If some of you have similar stories to tell, it could be interesting 
PS: the second slave is under planning stage
. I already have a SUGO SG05B with 450W Bronze power, an Antec Kühler H2O 620. I now have to decide for a mini-ITX mobo this time, with Wifi, and the CPU. Ram will definitely be 8Gb too, and a small SSD too. It'll be smaller, hopefully even quieter. The aim will be to stay under 400€ so that all in all, I will have two slaves for less than 1k€, running 12 threads at 3.2GHz at minimum. And it will for sure be less expensive than a big rig with an hexacore hyperthreaded processor. Unless proven otherwise 
