These are my specs:
Sigma Gaming Unicorn Mid-Tower
Asus P5Q Pro Intel P45 CrossFire Chipset
Intel® Core™ 2 Quad Q6600 @ 3.0GHz
Stock Intel CPU Cooling Fan
ATI XFX 4850 1GB
Thermaltake 600W power supply
My current CPU idle  is 43C, load is around 54C with a voltage of 1.37.
I'm running my Q6600 @ 3.0Ghz with my stock Intel cooler.
Is this fine or should I lower it?
Hm looks fine to me
The temperature is acceptable, but I think it wouldn't hurt to get an aftermarket cooler. The Arctic Freezer 7 might be a worthy investment.
Would any of you recommend the Xigmatek Dark Knight?
Yes.
Yes.
yes. cheap and good
Absolutely. That thing is aesthetically pleasing as well as effective in cooling in many many ways.
Yes, + like a million times.
What about the Core Contact cooler? Isn't it about the same price?
Nah, fuck it, get a Dark Knight.