Another one!!
I haven't checked, but off the top of my head maybe a 2 - 2.3ghz, but really.... what difference do you think a fairly minor increase in CPU clock speed will make to the overall usable speed of your computer...... probably hardly any at all!
You'd be better off saving the money a new CPU would cost and putting it towards a whole new base unit with faster everything!
(p.s. the CPU you have at the moment is considered to be one of the most overclockable ones in it's range, with extra cooling (and some knowledge and the correct motherboard) it's been clocked upto 3.7ghz!)