You're at least four generations behind.
The only LGA775 Socket CPUs that are faster than your current Qx6850 are the QX9650 and QX9770. That's if you can find any.
That doesn't mean that the QX9650 and QX9770 are supported by your motherboard's BIOS.
If you're looking at spending that kind of money for just a CPU you're better off buying new modern CPU, motherboard and memory than pouring it into obsolete end-of-life technology.