mike_s104,
Since everyone's mileage varies, for me the Diamond still isn't tailored the best in the world for 800 MHz.
If you do not specifically need an omni directional antenna, you could most likely benefit more by going with an 800 MHz beam/yagi/Yagi Uda antennna (won't say for certain since I don't know the distance, terrain, source, system(s) you're wanting to monitor).
If you do want an omni directional for 800 MHz, you can go a less expensive route with an AntennaCraft 'Scantenna ST-2' which most places sell for well under $50 including 50 feet of cable and shipping. All you'll need is one F female to BNC male adapter (unless you need more than 50' of feedline, or if you want to utilize any existing cable that you already have and that you haven't mentioned having (yet)).
If you've got more questions, ask away.