You can find performance reviews that have been posted on MR, TUAW, Ars Technica etc. comparing the models, but in general with processors, speed increase with no other changes is a linear (vs. geometric) performance increase, i.e. 10% faster speeds means 10% faster performance. So you can expect roughly a 12.5% increase in the maximum performance of a 1.8GHz processor over a 1.6GHz with everything else being the same. Keep in mind, though, that the processor often isn't the limiting factor - often it's the HDD speed, the amount/speed of the RAM, the speed of the bus, or the software's efficient usage of system resources and threading. So in practical use you'll likely only notice the difference if you're doing things that have high processor usage. If you're just browsing the web, sending e-mail, etc. you likely won't notice much difference. If you're rendering video or using CS3 and the like you're more likely to see the performance gain.
That all being said, there's always the future proofing argument.... A few years down the line when base system requirements have continued increasing and more software is taxing your system at a higher level, you might be happy to have the extra processing speed.