11 Jan 10 2:51 am
Ideally you want to keep the focus of your landing pages fairly narrow - if its too broad (ie youre chasing too many keywords) you risk diluting your keyword relevancy and getting a poor quality score from Google (pushing up your CPC - cost per click).
Choose one main keyword based on factors such as traffic volume and CPC. You'll have to decide how much the advertising is worth to you in determining this but obviously the keyword with the highest traffic is more likely to get impressions and thus click-thrus, and hopefully then sales.
The ad group is more about widening the net and doesnt have to be expansive, particularly if youve chosen a broad match for your main keyword as Google will automatically display your ad when it feels it's relevant.