Backpack - I have an Osprey Aether 85 and love it. I use it from one-night trips to week-long trips. You can always cinch the pack down, I would err on the side of getting a bigger pack. 50-60 is pretty small, I would have a very hard time packing everything I need for a multi-day trip into that. Once you decide on a size, the most important part is fit. Sure you can adjust packs but certain ones WILL fit you better. Fact. Buying a pack is like buying ski boots. Go to a shop and try them on (with some weight) and see what fits rights. I tried mine on and instantly knew it was the one. It fits so that 90% of the weight is on my hips with very little weight actually on my shoulders. Having it match your torso length is the secret.
Sleeping Bag / Pad - I love the Big Agnes sleeping bag and pad system. I currently have a North Face Blaze 20 degree and it does great, but the Big Agnes is so sick. The bags don't have any insulation on the bottom, instead the sleeping pad slips into a sleeve on the bottom and acts as insulation. That means two things: 1) the bag is lighter and packs smaller due to less material and 2) you won't roll off your pad. I would recommend a 20 degree bag for most three season camping. If you get one that is too warm you can always add a liner to the inside for colder nights.
Tent - Shop around and see what fits your needs. Keep in mind that when they say a "two-person" tent that means two people and nothing else. My girlfriend and I sleep in a Sierra Designs 2-person XL tent that has some extra length and width to it that allows our dog to fit inside without totally being on top of us. Bags go in the vestibule. I would recommend a matching footprint made by the manufacturer. Find a tent that has the features you want and is on sale. Keep it under 5 lbs if it is a backpacking tent.
Other stuff - I have two other items that are the best thing ever when backpacking: 1) CamelBack water bottle UV filter. Not packing fluid is key to save weight, making a water filter a necessity. This one works great, is super light, and is idiot proof. Get the pre-filter to screen to catch any larger items before filtering. 2) JetBoil -this makes camping so easy. Nothing is worse than struggling to get a good fire going in order to eat. You can boil water in a jet boil in, I kid you not, two minutes. Whoever said a watched pot never boils never had a JetBoil. Bring some Mountain House dehydrated meals, oatmeal, and coffee with you and you are set.
There are always other little things to pick up along the way but these will get you the necessities. A good knife is good to have, I always carry a compass just in case, chocolate bars are delicious and contain a lot of energy. A good headlamp can make your life nice. Hit me up if you have more specific questions and I'll be happy to answer what I can.
Also, forest service cabins make for awesome backpacking trips... http://www.recreation.gov
to search for ones in your area.