If you don't reload I would definitely stay with common cartridges. 30-06, 300mag, 280rem, 7mm08, you will have a much better chance finding factory ammo that shoots good from your gun.
Before you take the plunge and get a screemer, take a look at some ballistic tables found in Hornady, Barnes, etc.. reloading manuals, it will show you just how minor the trajectory is affected by an increase of 2-300fps.
A quick example would be a 160g Hornady, Ballistic tip. Fired @3000 fps, and sighted dead on at 200yds, it will drop 6.3" at 300yds.
Now that same bullet fired @2800fps. and sighted dead on at 200yds will drop 7.4" at 300yds. For a whopping difference of 1.1"
At the expense of buying "magnum" ammo, ear splitting muzzle blast, and unnecessary recoil? that is why I suggest a 280rem, 7mm-08, 30-06 they are popular and almost always available. But if you feel the need for speed then the 300mag gets the nod, ammo is plentiful, and it is more than fast enough.