Farmview - you're getting some wild answers here.
Build your good rifle, scope it well, build good ammo for it and practice with it. Learn the "come ups" on your scope and the windage adjustments too. Windage is the big bugaboo of long-range shooting. Trajectory is pretty easy to deal with once you've built your drop chart.
At longer ranges you'll of course have reduced velocity, so consider one of the "softer" bullets like a Berger VLD or the Hornady A-Max, both of which have proved themselves time and time again in long range hunting situations. You want expansion and these bullets tend to expand very well, even at reduced velocity. They also tend to be very accurate.
I've gone 400 yards on deer with a factory built, sporter-weight .25-06 Remington, the flat trajectory helps considerably. Beyond that, I'll rely on a purpose built .308 Win, which is very accurate and which I've been shooting for about 14 years now. Long enough to wear out several .308 Win barrels, which takes 4000 -6000 rounds each. Long enough to earn the NRA "high master" rank. Long enough to shoot some 2.5" groups out there at 600 pretty consistently. Also long enough to totally blow a shot now and again! ;D
A good scope with target type turrets is really important - so you can simply range your target, dial the appropriate elevation, deal with the wind, and send the bullet. It's not rocket science, but it does take some "homework" as Rustystud put it.
Deer aren't that hard to kill, if you can get the danged bullet into the chest and if it expands, causing lots of damage. I use a .308, but other cartridges will offer you a flatter trajectory for sure, as well as reduced wind drift. Someday I may change it to a .260 or a 7mm-08, but probably not for a good long time as I've got a mess of .308 brass & bullets on hand!
Try over at: http://www.precisionlongrangehunter.com/forum/ubbthreads.php?ubb=cfrm for some interesting discussions of long range hunting & shooting. Some of those guys put my self-imposed 600 yard limit to shame!
Regards, Guy