What's the cheapest, low power micro atx and cpu setup? It doesn't matter if they are older and not in production anymore, maybe i can find them on the second hand market. I want to make a cheap, passively cooled media PC, just for music, so i don't need powerful cpu and video card...

S754 Sempron + undervoltable mobo with onboard graphics.

Find yourself an old P1 and take the fan off

passive.. do u want completely passive ie no fans atall? or.. just no cpu fan?
music.. mp3s? aac? flac??

mp3s should be fine on any p1; i think AAC will need >150mhz. flac i don't know, but considerably more i'd imagine.
The problem with a P1 i'd guess would be finding a microATX one? as many P1s were AT rather than ATX and i'm not sure how many micro-ATX boards existed then (?). with a low p1 if u just want mp3s then u could literally just yank the fan off the cpu (or case) and the psu.. (obviously other factors affect)

then next up is p3, and celerons based on the P3. celerons from 533/566/600 mhz had variants which were 1.5v which are the lowest power afaik in that range.

..above that theres mobile P4-m, which u can stick in any (preferably undervoltable) s478 mobo.
or a geode NX (eg 1750+), which u can stick in any (preferably undervoltable) socket A mobo.

Then u have your s754 sempron, 90nm, 2600+ at the high-end, widely available new.

OB gfx all-round; but if u cant get a board with onboard gfx then its really not a problem; just get an old 8mb PCI card that can display the res u want.

..then add a CF>ATA adaptor, CF card for OS (shouldn't need >128mb) +wifi and stream music wirelessly from somewhere else would be the best bet. if u don't want to leave your PC on 'serving' then u could just make a server in another room with another similar system^.

The pc will have 2 fans (one in the psu@6v and one at the exhaust @6v) in an Antec 2400 case...Or maybe use a Pico PSU, i don't know yet. The mobo, cpu, hdd, video will be cooled passively.
Will play mostly flac files with kernel streaming enabled (maybe).
Oh, and it must have usb 2.0 ports.

If i choose a sempron 2600+ 754, i don't know what mATX mobo should i get.

Undervoltable Motherboards

U-voltable S754 boards not on that list include Asrock K8Upgrade VM800, Biostar Tforce 6100 (mATX), Winfast Foxconn 760GXK8MC-RS (mATX) and I'm sure there are others.

my vote is for the biostar tforce 6100 board.

Here's something to think about.
You get an OEM S754 Athlon 3400+ Venice with a free Gigabyte K8M800 mobo. That's $99.00 for a mobo, CPU, and video.