After you've got your bike, the next question is how to carry everything.
Some people swear by trailers, some people swear by panniers, most people say if you're completely undecided, only you can tell by trying both.
This is about panniers.
It's generally accepted that a bike is much more stable if you split the load between front and back. Sure, you can carry 60lbs on a rear rack only, but many people end up asking "Why is my bike squirrely when it used to ride so solid?" Rear-rack only is fine for shopping, for commuting runs and for light touring, but for loaded touring you should plan on using both front and rear racks/panniers.
Some people even say to go with 60/40 split, front/rear. That's 60% of the weight on the front. The effect is as if you're riding on rails, you have a very stable and sure-footed load. It will probably take an afternoon to get used to the feel, but after that there's no turning back.
Many panniers manufacturers to choose from.
Ortliebs,
Arkel,
Jandd (and others) are recognized for top-of-the-line bags. They're well-designed, rugged, last for multiple round-the-world tours. No small factor is that when something does go wrong with them (say, a mounting hook comes loose), they are quick to respond. If you're headed to Patagonia or across the Silk Road, consider these expensive panniers only. Others will certainly recommend their favorites.
But for touring in areas that aren't so remote (say, across the US), you can get by just fine on panniers that'll save you a bunch of cash.
Nashbar,
Performance,
REI (MEC in Canada) -- all sell good panniers starting in the $50 range (one pair) that can last over many summers and thousands of miles. My wife and I are proof: a set of REIs and a set of Nashbars that have 40,000 miles on 'em and still have several years left of service. Plus, for shopping trips around home, the less expensive ones are ideal since they don't put such a big investment at risk.
Only you can decide how important price is in choosing equipment. One person may argue to "Go for the best at the start". Another person may advocate for Nashbar panniers, based on price and knowing that they'll probably last long enough for their use. If cost is a consideration, you can still get good quality for your budget. If your wallet allows, go for expedition quality.
-- Mark