A few other sites to check out :
www.farecast.com ~ They track flights and then give you an idea on price history whether now is a good time to buy or if you should wait. You can even purchase a guarantee through them for (I think) $10, but if you book direct through an airline, you can usually get a voucher for price difference if the flight becomes cheaper.
Which leads me to :
www.yapta.com - It's a firefox r IE addon where you can tag flights from certain websites and it'll track the price for you. If you buy a flight, you tell it how much you paid and it'll notify you if the price changes to something lower so you can try and get a refund of some kind.
Here's a screenshot from Yapta of a flight I purchased and it being tracked :
