Much much better.
Judging from the GHz and RAM of the Vista computer, this is a rather old computer, especially since it is running Vista. 1.60GHz, would not even compare to Apple's 1.4GHz Processor in the Air.
So, from 1.60 GHz to 2.7 GHz is about 10 times better than your other one, and then the fact it is an i5 makes it even better. 8GB of RAM, will definitely be an improvement.
And then Mac's tend to run better than PC, so a 2.7GHz i5 on the
iMac will run better than a 2.7GHz i5 on a PC. Since Mac's take every bit of the processing power to give you quicker performance. That's the only reason Apple can pull off a 1.4GHz Processor in the Air and have it run amazingly.
I hope I helped! Expect a dramatic change in performance and speed, that's all I can say.