555 chip

[setishock]

In the video Dave did on the 555 chip where he came up with a PWM controller for his magnifier lighting, I came up with a couple of questions. Let me back up and explain why.
I have an EVGA Nvidia GTX650 video card which has an awful loud fan at higher speeds. The PWM off the card modulates the ground side of the power to the stock 2 wire fan. There is no PWM or tach leads on the stock fan. The replacement will have a 4 wire fan and I'd like to have the PWM system in the fan adjust the speed and the tach lead confirm the speed to the controller chip. OK having explained all that...
1> Is it possible to replace the pot with a 10K thermistor to regulate the PWM output?
2> In order to get a positive going PWM signal to the fan, which configuration in his 2 diagrams would I use?

As a side note, I think the card uses a TI LM63 or LM64 chip. I can do major brain surgery on the card to modify the way the chip is wired up and make the onboard chip do like I want it. But if I can stick a thermistor to the under side of the heat sink and use an external PWM controller, it would keep me out of the OR with the card.