I read alot about this mod and did it last week. The lcd would not work properly witha 6800gt so I picked up a mystique on ebay like you.
Does your lcd turn on when you power your pc? Mine didnt and I had to wire 5v power to it from a tranny on the back.
Once I did that, I installed powerstrip, changed the res to 640x480i, made sure interlaced and csync were checked. To adjust the timings I just used the up/down arrows next to the pixel clock until some sort of image appeared. Took 1 minute tops to tweek it. Below is a pic. The rocker switch above the lcd is to turn it on/off when not in use.
Feel free to ask any questions. Btw I'm using the matrox mystique 1064sg. some mystiques dont work from what I've read.