I can't seem to overclock my 3500+ Plus, if I did, it'd probably overheat being at 49 degrees C at load already, and thats slightly underclocked from default settings to keep the system quiet.
Good luck with it. I hope you can push a bit more out
Try raising the FSB a little then run it to see if its stable, if you want to change the voltage then google overclocking there are plenty of websites that tell you how to do it but just be careful you don't fry your CPU.