I used a XigS1283 w/o crossbow bracket and it kept my Q6600 G0 1.3000VID @ 3.6GHz (400x9)/1.42v cooled for months. Usually load between mid-50's to lower 60's on cores, which is well within TJmax specs for this chip.
I now have a crossbow bracket and wasn't really all that impressed with it, for me it didn't seem to hold any stronger or tighter than the intel clips, also didn't lower my temps any. But YMMV here, some hate or have issues with the intel clips, I've dealt with them for enough years I have no issues with them. But one thing that made a difference was either a 2nd medium flow 120mm fan or a single more powerful fan. Either rate, a plain S1283 w/o any extras will cool a Q6600 just fine, now as for getting beyond the 3.6GHz range, it depends on the chip really...some flat out won't go any further than a certain speed. So ymmv there too, but I would highly recommend the XigS1283, bang for the buck it's a damn fine cooler and one of the best out there still.
