Cpu query

Idle temperatures don't matter much, as long as you are less than about 10C above ambient temps it is doing its job well. What is more important are load temperatures, I would load it up with Prime95 to see what temperatures you hit under max load as those are the temps that can actually cause harm if they are too high. Try to keep it under 70C ish.

Are you looking for a new heatsink to overclock with, or just to replace the stock intel heatsink? The "best" heatsink is also very expensive and won't provide you much advantage over a decent heatsink if you aren't going for massive overclocks. The cooler master hyper 212+ is pretty cheap right now thanks to its MIR, its related coolers are also reasonably well priced and perform fairly well.
http://www.newegg.com/Product/Product.aspx?Item=N82E168...

+1^
Here's the specs for your Q9550. http://www.cpu-world.com/CPUs/Core_2/Intel-Core%202%20Q...
Arctic Silver 5 or Arctic MX-4 would be two good TIM choices.
http://www.newegg.com/Product/Product.aspx?Item=N82E168...
http://www.newegg.com/Product/Product.aspx?Item=N82E168...
The latter has the added benefit of being non-conductive.

I have the Arctic Silver Céramique 2 and it does quite well as well.