You do not need to go for the expensive Gortex. Particularly in the Sierra. Just get a coated nylon rain jacket. I would not rely on those flimsy plastic "emergency" jackets - they do not do well in a wind and tear easily. These jackets will make you sweat if you hike in the rain, but the chances you will have to hike in the rain in the Sierra are minimal.

You want to get good socks because bad socks will cause lots of foot problems. I recommend wool. Shoes also should fit well. This does not mean expensive - just be sure they fit with the socks you intend on wearing. I would do a long (10-mile) hike at home in the shoes and socks you are going to use to be sure they are OK - and break in the shoes. You do not have to do big expensive boots. I often use trail runners in the Sierra. Some stores, like REI, sell slightly used gear - they have an annual sale, although I cannot remember the exact date they do this.

In the old days we would hike in shorts, put on long underware when it got cold (under the shorts) and then put on wind pants when it got really cold. The wind pants also could be worn over the shorts. The long johns were used for pajamas. This combination still works even if it looks a bit dorky. Lots of cheap wind pants are available as runner's gear at Wal Mart.

Although you may want to buy cheap when you get started, if you intend on sticking with backpacking, splurge each year on one good quality item and soon you will have better gear without one big credit card debt. In high-tech gear, expensive usually means lighter weight. You pay dearly to shave off ounces.