Thanks for the good info, esp. on PWM wire colors and mappings. Here's me trying to give back a little to this forum:
Can't find adapter cable
I also failed to find an adapter cable (4-pin "mini PWM" plug to 4-pin standard fan PWM socket) online; I tried newegg, digikey, buy dot com, nextag, etc. I considered cutting the white plug from the stock cooling fan to solder into a custom cable, but I wanted to keep the stock cooling system intact as a backup if silent cooling didn't work.
Sound Blaster audio cable(s) to the rescue
I found that a Sound Blaster (SB) internal audio cable, the kind that shipped with virtually every sound card and CD-ROM in the early/mid-1990s, has a white plug that fits the mini PWM jack. The other end of the SB cable is a black 4-pin square pin connector ("4-in-a-row"), identical to a PC speaker's internal connector that attaches to square pins on a motherboard.
Adding a 4th wire
The only catch is SB cables have only 3 wires. To get the 4th wire, steal a wire from a second SB cable: use a small flat-blade screwdriver to pry plastic clips and pop out the wire along with its square pin headers on both ends, then slide in the headers into the empty slots on the original SB cable to complete your 4-pin adapter cable. No soldering required. The only caveat is the cable has excessive length.
The white plug goes to the GPU board; the black plug fits snugly into the new PWM fan's 4-pin "extension socket" (I have an Arctic Cooling 12PWM with shared PWM). With all 4 wires connected, the video card sees fan control and sense lines, so it can control fan speed based on temperature.
Sound Blaster cable suppliers?
If you don't have any SB cables lying around, you can get 2 of them shipped for $2.38 (total for both cables, including shipping) from "Monoprice dot com", part #694, "Sound Blaster/MPC-2, Audio cable". Disclaimer: I've never ordered from Monoprice, so I don't know whether they're reliable
At least you can search other vendors for "Sound Blaster audio cable" or "MPC-2".
Hope this helps.