You have to make sure the you put the mobo back together properly. And make sure the the ribbon from the clip is not getting crunched at the same time.
If the GPU and CPU are not seated correctly on the heat sinks, the PS3 will not boot.
As for the stripped screw, try to use a bigger bit, with some pressure to unscrew it.
You should not be using power tools on electronics. You need a set of precision screwdrivers. Never use a power screwdriver!