GENERAL THOUGHTS: Most people who are into backpacking these days swear by Gore-Tex. I think it's crap. It's twice as expensive as the same thing made with different material, and it doesn't work as well as it was designed, anyway. All that "pores large enough to vent water vapor but smaller than water drops" stuff is fine and dandy until you get it all muddy. Then what good does all that expensive stuff do? Try a different approach. One way or another, you are going to get wet. Instead of wasting all your money and energy on trying to stay dry, which will never happen if it rains, focus on drying fast. Down is a great insulator, but it doesn't dry very fast at all. Try fleece, sometimes called Polartec. It will keep you almost as warm, and dries much faster. Instead of Gore-Tex, go for a simple rubberized nylon outer garment, if you really have a love affair with some sort of ultra-light thing. They will be much cheaper, and will work nearly as well as all the expensive stuff. If you don't mind the weight (which I don't) get an oilskin jacket. Oilskin is nearly indestructible, too. Here's another tip for warm weather hiking: get yourself a good long pair of swim trunks to wear while hiking. It is much lighter than carrying cotton underwear and a pair of shorts, and it has its own built-in underwear. Of course, this doesn't work so well if you aren't in temperate weather.

GENERAL TIPS:

8-POCKET EMT PANTS: Consider these for your hiking and backpacking needs. They can get kinda spendy, but for about $80 you can find them online. Military BDU trousers and cargo pants are nice, but EMT pants have cargo pockets and additional pockets on the calf of each leg. Count the pockets: 2 hand pockets, 2 back pockets, 2 cargo pockets, and 2 calf pockets. There are some with the ever-useful change pocket and are called 9-pocket pants, but have you ever really used the change pocket? No really, have you? Ever? Another nice feature is that the right cargo pocket on many is sectioned for organizing EMT goodies, but you can use it to organize various instant-need gear, such as flashlights, knives, snacks, first aid kit, etc. Pro Tuff is a really nice brand, though they can get a little up there in price.

CHRISTOPHER'S TRIPLE LAYER CLOTHING SYSTEM: I have been backpacking for 11 years, now, and 4 years of that has been through some pretty rough hikes. I've been hiking through jungles in the Phillipines, I've been up the side of an active 14000 ft volcano, I've camped in valleys of tropical rain forests, and I've been in knee deep snow. Over the years I have come up with a sort of three layer system that I like. The idea is as follows. All three layers put together are a great cold and wet weather system. They will keep you fairly dry and warm. They can also be switched around for your particular ecosystem-specific needs. Layer one should ALWAYS be worn, for example if you are in a high desert where it is cold and dry, wear layer two over layer one, but never just layer two. If you are hiking in the hot and wet Southeast Asia jungles, then wear layers one and three.
LAYER ONE: This layer is the closest one to your body. It consists of extremely lightweight material, and the focus is on comfort. These clothes are lightweight, and they dry very easily, because when you sweat these are the clothes that are going to get soaked. Examples: light swim trunks, t-shirts, and socks. Try to avoid cotton. It breeds icky things, and it doesn't absorb moisture very well. Polypropelene and nylon are good.
LAYER TWO: This layer is the primary layer for warmth. It should also be of a material that dries easily, because this layer is sandwiched between two layers that are going to be wet often, depending on the hiking conditions. This layer should nearly ALWAYS be long legged and long sleeved. Good stuff for this layer is fleece, Polartec, and Gov't Issue Polypro Thermals.
LAYER THREE: This layer is your outermost layer, and hence is the layer that should be the heaviest (to protect you from the bush) and the most water repellant and wind proof (to protect you from the weather). Oilskin is excellent, though good heavy blue jeans are great, too. The main problem with jeans is that they don't deal well with moisture. They tend to soak it up and stay soaked. Like layer two, try and keep this layer long sleeved and legged. That way the heavy material will protect your arms and legs from scratches and bumps and dents.

RAIN CLOTHING: I personally like oilskin. Yes, I realize that it is pretty heavy, as far as backpacking equipment goes, and yes, I realize that it isn't even true "backpacking" clothes, but based on my experience, you will never find a material that is (#1) as heavy and durable, and (#2) as warm and comfortable in the rain. It will last a long time, and it will keep the moisture out. I go for the long drovers with a small cape around the shoulders. They are simple to clean (manufacturers state that to clean it, you just hose it off then let it drip dry), they are breathable (hey, it's cotton canvas!), and THEY ARE SILENT!! I can't stand those dumb PVC, or Nylon, or Gore-Tex, or rubberized cloth, or vinyl suits that sound like a hollow tin roof in heavy rain. Ah! Drives me nuts! Oilskin is quiet. Oilskin, by the way, has an interesting history. It supposedly started in the early 1800s, when some captain decided his sailors needed better protection from the stormy seas. He took the extra sails carried on board his ship, which had been soaked in linseed oil, and made a bunch of capes for his men. The sailors loved them, and it caught on and became a popular sailor thing. People on the docks started hearing about it, and after trying it themselves, thought it was cool. Oilskin caught on, and became used by everyone. It eventually transformed into a more practical (and fashion conscious) jacket, with a cape over the shoulders.

NYLONS: If you are a guy, find the largest size you can, because while women are smaller than us, they are even smaller then they actually look. They are great for keeping warm. Try it, sometime. Don't walk around in an itty bitty skirt and show off your legs, but some day when it is cold out, forget the thermals and go for nylons instead. They don't work quite as go od , but as far as size and bulk goes, they barely even weigh anything, and will fit anywhere (except the useless 5th pocket in your jeans that'll barely hold a paperclip). They are also good for keeping leeches off your legs when crossing rivers, or wading into lakes. WARNING: don't just yank them on. Put them on like a sock. If you pull them on, they will catch your leg hairs and hurt like the dickens. For all you paramilitary commando-wannabes, they can also be stretched out over your binoculars and rifle scopes to hide the glare. With the focal point of your vision so far away, the shear ones don't hinder your eyesight at all.