DO NOT GO OVER 3.5 volts or you could fry your Mother board your Memory or Chip. I would take the V-core is 2.7 volts. But try it with out changing the voltage first, You never know it might work. If you need more voltage you will know as you will get errors loading windows. The higher the speed and voltage the more cooling you WILL need to keep the chip temperature down.
OK lets move on to what to try first.
The fastest way is to go for the multiplier first. The best over all total system performance increase will be seen by increasing the bus speed as it speeds up everything in your system. The main rule to follow is the AGP and PCI bus speeds go up with the FSB speed as in if your running your FSB at 83 MHz then your AGP bus will be running at 83MHz instead of the stock speed of 66MHz. Most AGP graphics cards do not like AGP bus speeds over 75MHz check your mother board manual and or there web site as to what the varying bus speeds are for, FSB to AGP, PCI are. Do some reading.