Hate to break it to ya, but there are no good mounting mechanisms for the LGA775 (Q9550) chips that do not require you to take out the motherboard. (Unless your motherboard tray has an open area behind the CPU!) All *GOOD* air heatsinks are heavy, and that requires the use of a backplate to be screwed in on the other side of the motherboard. I have a Scythe Ninja Plus Rev.B on my E6600 (same mount format as Q9550), and it uses push-pins.. but I wish it didn't, since it's more likely to not have a proper seat on the CPU's heatspreader.

My preferred air coolers:
Thermalright IFX14 (easily the top air-cooled heatsink in my opinion.. but huge and expensive)
Thermalright 120mm towers.. there are a few models.
Tuniq Tower 120
Zalman 9500+ series of copper pipe coolers..