I wouldn't upgrade if I was you - not enough of an increase in performance vs. price to warrant it.
If you have your motherboard's make & model, visit the manufacturer's website to learn what it's capable of (in terms of the 'best' CPU, I'd assume?).
If you don't know what make/model, one of these two utilities should tell you - Belarc Advisor click here or Everest click here. G