Intel core two duo e8400 oc

4.0, 4.2 Ghz.If you have a good motherboard you can go to 4.4 , 4.5Ghz.

it depends on your gpu too
if their is no additional graphics card in your system then you can go as high as you can
do keep an eye on cpu temps too and keep them bellow 65'C at full load, and use occt or prime95 to test stability

to overclock via software you can try Intel® Desktop Control Center (not sure if it will work)