So I've done a fair bit of backpacking before, but I've never owned much of my own gear. I've got a backpack (50L), a water filter, and a sleeping bag that needs to be replaced - that's about it. I'm planning on doing the JMT in early August, and since this will be the first of what I hope to be many serious backpacking ventures, I think I'd like to invest in some good gear. Here's what I'm thinking of getting:

-sleeping bag (0 degrees, comfortable as possible)
-tent (2 person, durable, easy, lightweight)
-stove (for the technologically inept)
-backpack (will 50L be big enough?)
-bear canister (but I may just rent one)

I've browsed zillions of stores for these items and feel a little overwhelmed by all the options. If you have recommendations for these, I'd love to hear them - especially if they're relatively inexpensive. I'm afraid I can't spend a ton.

Thanks!

You don't say whether you are planning on going solo or with a partner but your list of gear so far is a bit of overkill for either. Start some research by looking at the home page of this site and clicking on the 18 lb, 3 day pack. That is a good starting list for gear that will serve you for the entire JMT.

A zero degree bag would sure keep you warm but would be too heavy and too warm for what you would encounter. A 50 L pack is certainly large enough but can be heavy as well. The lightest tent is a lot heavier than any tarp. Water treatment chemicals are a lot lighter than a filter. Before you spend any money, do a lot more research and discover how to shave weight. You don't have to sacrifice much in the way of comfort to carry a light pack with complete safety. If you are at all handy with a sewing machine, you can save a lot of money.

I did the JMT last August; my gear load was about 15 lb including bear canister and a few comfort items. And, there are those who do it with even less. I also saw a lot of folks with packs that must have been 40 lb, not including food, struggling up some of the passes; some of them were bailing out. There is a lot of elevation to be gained and lost on the JMT; the lighter your pack, the easier it will be.

Doing the research is fun too!

I second what Pika has said. Most lightweight hikers of the JMT usually carry a 30 or 40 degree bag, a lightweight solo shelter, if going solo, and a lightweight pack. I'm planning an August thru-hike of the JMT and I've got my base weight down to about 12 lbs and still carrying a few luxury items.
Oz

To reiterated and add to the info already mentioned, a 0 degrees (fahrenheit) is a bit overkill for the JMT in summer. I would recommend something more like 20-30 degrees, and for some that still may be a bit too much. A two person tent isn't totally necessary if you are solo, unless you plan to spend a good deal of time in the tent and want the room. Tarps are an option, and I hiked the JMT with a homemade tart. That being said though, I wouldn't recommend heading out for several weeks if you aren't familiar with them - having knowledge of their advantages and disadvantages beforehand. I'd recommend looking into backpacks last, at the point when you know how much gear you will be carrying. I'd also recommend - if you have the option - to try out the pack with weight in beforehand. The last thing you want is an uncomfortable pack you must carry for 200+ miles. In any event, I'd recommend whatever you choose, test out and become familiar with the gear beforehand on a few overnight trips. Last but not least - have fun! The JMT is an incredible experience

I went through most of the JMT (PCT) in June, and so my experience was likely a bit different, partly because I was physically and mentally up-to-speed when I started that stretch, and partly because there's more snow in June, and perhaps less bugs (I saw almost none until after Tuolumne meadows).

A couple of notes regarding the bear can --- you mentioned possibly renting one. This might be a good idea. I own a bear can, and so naturally used that (Bear Vault). Mine's a fine can, but a fellow hiker I was with for part of the JMT portion felt happy to have rented the lighter weight Bearikade. http://www.wild-ideas.net/
OTOH, it's not cheap to rent; you could almost buy a different brand of can for what it costs to rent. I liked my bearvault brand, both for the product itself, and their outstanding customer service. http://www.bearvault.com/

Of course the best option of all is if you have a friend that can loan you one. It's a bit of money to spend and then storage space at home for something that very rarely gets used. Seems to me that there might be a rental program at one or more of the ways in to the JMT, but I'm not sure on that.

One final comment on the bear can, especially if you end up renting one locally (local to the JMT): think ahead of how you plan to carry it. Some people carry it inside the pack, some strap it on the outside. I started with the latter approach and switched to the former, but that was a challenge as I didn't use a large volume backpack. It's also worth thinking through how much food you plan to pack in it, based on estimated caloric needs and where you expect to resupply; the types of food you pack are a factor (how compressible).

The Sierras truly are awesome (not the George Bush type of "awesome", but actually invoking a sense of awe). I definitely want to go back!