You've got a great platform from which to start. Your choice in caliber will certainly take a deer at 500 yards (really, it has capabilities to 1k and maybe beyond - on deer)
My suggestions for you....
1. Take the rifle out and determine its accuracy potential. If you think it can do better, you might want to consider bedding the action.
2. Ammo choice - 160grain bullet is a bit light for the 300mag. Find a 175 or heavier bullet with a good ballistic coefficient. The heavier bullet will carry more energy at long range and is less affected by wind.
3. practice, practice, practice - while you have equipment with the capability to shoot long range, it's no better than the guy on the trigger. So practice shooting at 500+ (preferably longer than 500, because that will make a 500 yard shoot seem easy). Intially, settle down and take your time with the shooting. Once you become more familiar with your gear and proficient with the distance, add some pressure to the shooting in some way. Remember, when the buck steps out, you won't always have the luxury of a lot of time to take the shot.
4. Other equipment
Range finder - get one that can range the distance accurately.
Spotting Scope - quality glass is always a good thing
Weather Station - Kestrel
makes great equipment, Caldwell makes one that is sufficient
Ballistic Software - exbal, JBM - plenty of choice you can buy or use free on the internet.
Have fun and enjoy the addiction!