It's very difficult to say if this upgrade will work. You will need to flash your bios first, and then try and see.
from the little bit of research I've done, it looks like the E6700 does work but I can't find anybody attempting to use an E 6850
cpu upgrade pavilion a1720n - HP Support Forum - 623019
the E6300 has a code name Wolfdale and the E6850 has a different code name Conroe, which could be an issue
If this is your only pc, then I can not recommend you "experiment" on this. Flashing your bios can brick (kill) your motherboard if something does not go right, and there's always a possibility of causing the board to fail just by handling it alone. Live with what you have and save up to get a new pc and then you can afford to experiment with this pc