I haven't been keeping up with computer tech, but, the clock speed (measured in Ghz these days) use to be the most important thing. Right up until the end of the Pentium 4, it was all about clock speed. You wanted the fasted clock speed, period. But these days, clock speed doesn't play as important of a roll and for a couple of reasons.
The biggest reason is how many cores the CPU has. If given the choice between a 3.2Ghz Pentium 4 and a 2.4Ghz Core 2 Duo, I'd go with the Core 2 Duo hands down. The difference is even greater with the i7 because it has FOUR cores (or even SIX now I think), among other things.
The other things that come into play are things like cache size and hyper threading.
http://en.wikipedia.org/wiki/CPU_cachehttp://en.wikipedia.org/wiki/Hyper-threadingAfter reading what CPU cache does, you'll realize why the more the better. The i5's are the mid range CPU, compared to the i7's. IIRC, they have less cache. As for hyper threading, the i5 doesn't have hyper threading while the i7 does.
Another reason why clock speed doesn't matter so much is overclocking. Back in the P4 days, there wasn't a lot of overclocking options. These days, basically any motherboard has OC options, and the i7 CPUs can overclock a LOT. I don't recall specs, but I do recall reading large increases when the i7s first came out. Something like going from 2.6 (stock) to 3.2Ghz on air.
Also, I don't believe there is a 1.8Ghz i7. When the i7's first came out, the slowest clock speed was 2.66Ghz. IIRC, there isn't even an i7 on the market now with a clock speed below 3Ghz. But long story short, if I had to choose between the i5 and i7, I'd go with the i7.