Until today I considered only to change CPU, Motherboard, GPU.
But in my budget I can only get either a i3 2120/2130, or a Phenom II X4 965, and another user told me I am better off with the i3.
My main use for the desktop is autocad, but I will also play Mass Effect, Dead Island, etc... if possible on high settings.
Can you tell me if it is advisable to instead buy a new LGA775 motherboard, and overclock the Q6600, or I´m better off with a new processor? ...and save a few euros!!.... My new Gpu will be a EVGA GTX 560 Ti
I would wait about 3-4 months until the Ivy Bridge i3's come out in the very least. I think they will offer better stock performance since you can't overclock them. Your CPU will be fine for Mass Effect, Dead Island, etc. I don't see any CPU intensive games on there.
Also, downgrading from a quad core to a dual core is undesirable even if the performance/core is much higher. I would try to save up for an i5 if possible especially if you're going to use Autocad..