your e8400 has new stuff like sse4 I believe and is 45nm. q6600 is old gen 65nm cpu. You should look for something like a q9300/q9450/q9550 if it is compatible with your pc. I have a q9300 machine with 6850 and it goes well with it.
My 4 year old laptop has a more power graphics card then your desktop and that is a basic mid range 9600m gt 512mb gddr3 one in my 5930g. IMO you only need to upgrade your gpu to something like a 7750/6770 to notice big improvements and your cpu should cope with these gpus.
As i said in the description i'll be upgrading the graphics card to a gtx560 or 6870/6950.
You will do just fine with Q8300. I can play BF3 without any problems...also with other games such as ME3, Skyrim, SC2 and Torchlight 2
The most important thing is, just do not forget to upgrade also your GPU.
Do you plan to get something new or second handed?
I would not go for a new Q8300 or anything with LGA775.
Try to get used ones from e-bay.