I got a 2.2ghz up to 3ghz
Its still going at that speed now 4 years later.
with certain chips the only difference between the mid range and the top range is the stock cooler and the voltage and multiplier settings.
Shop around, and see what people are getting result wise and its safer than you would think
#edit#
Forgot to say that its dual core.. so thats double the increase
AMD 4200+