Ok before I say anything, I want to give my disclaimer: I am not a long range shooter, have only ever shot at 1000 yards one time, fired a grand total of two rounds, only one of which hit the target. That said I have a pretty good grasp on ballistics and reloading. Just take everything I say with a grain of salt...
Ok with disclaimer out of the way, here is what I'd do:
Since you are most likely NOT going to shoot bad guys or animals, just targets, you probably don't need .30 caliber. I noticed serveral other members suggest 7mm or even 6.5mm, and I think that's a good idea. I personally am a big fan of .280 remington (basically a 7mm version of .30-06). A 160gr or 175gr bullet in 7mm will have as good or better BC than a heavier bullet in .30 cal would. Basically you'd get similar preformace in terms of trajetory but with less recoil. You will get a little less foot-pounds of energy on the receiving end, but that shouldn't matter if you're not hunting.
I know your counter for this argument is "but I already have a bunch of .30-06". Well unless that .30-06 ammo you've got is "match grade" stuff, I doubt you'll use it at that distance anyway.
Also somebody in an early post mentioned the Sniper 101 series on youtube. I've watch some of it (not the whole thing because it is very long), and it's good info as far as I can tell. Here is the link:
http://www.youtube.com/watch?v=pwG-D0HjCBQ
Last thing I will mention, and others have said this too, is that you should get into reloading if you haven't already. You can really customize your loads to your particular rifle if you reload, this will be a big help I would think...