It might make you unpopular with the neighbors, but Federal law
prohibits restrictions on outdoor antennas:
http://www.fcc.gov/guides/over-air-r...n-devices-rule
db gain is the magic number to look at. More is better, absent other considerations like size, weight, cost, and angry neighbors. There's an added wrinkle of dbi vs dbd. Google can help you there, though:
http://www.digi.com/support/kbase/kb...ltdetl?id=2146
The bigger problem is that it's often referred to as just 'db', and you have to hunt around to figure out which one a particular manufacturer is using.
A preamp can certainly help if you've got a long cable run from the antenna or splitters to multiple sets. As many here have said before, however, it can't create a signal where there isn't one. It can only work with what it gets from the antenna, so that's why db gain is so important. Ideally, you want a preamp with a low noise figure, 2-3 db.
A good first step is to look at your TV Fool report and decide which stations you really need and which ones are 'nice to haves'. That's going to drive your antenna choice, preamp, and mounting choices.