I think this is all going to work and I hope to be able to post 4 GHz screenshots soon!
With the right ambient temps, and a decent 6600 chip, 4+ghz shouldn't be a problem; I've bench tested my own 6400 at over 4.2ghz with the very same pressure mounted infinity.

By the way: What cleaning stuff do I have to use for cleaning the connection points of the cpu to the socket?
Because after lapping my cpu the bottom might become dirty too.
Dry sand the CPU when lapping it; wet sand works better for the Infinity.

When doing the CPU, if provided when you purchased it, use the little plastic cap that goes over the back of the chip; otherwise, when finished lapping the CPU, just blow the pin side out real good with some compressed air. Also, you can use a magnifying glass to inspect for any particles.

I wouldn't recommend any sort of cleaner, however ... although it might not hurt if you use a high alcohol concentration cleaner and let the chip dry adequately.