I've been reading tom's hardware forums for a long time and have come to value and respect the opinions and comments of the posters quite a bit.
My wife and I will be building a new computer for her over the holiday season. We are on a somewhat tight budget, which has made this an interesting and fun challenge for us. She will be using her computer mostly for simple web browsing, streaming videos, playing/downloading music, and some light gaming such as WoW and Swtor. Not including the cost of windows, I'd like to keep the cost of the tower to somewhere between $400-$500.
Get a 500 watt psu, you need less but it leaves some future upgradability, and it's probably around 50 for one, go with a low end mid tower, ultra makes one called the blaster, and it's 30 to 40 and i've used it in builds before, it's a good enough case. Buy a 20 dollar DVD drive from lite on, they work great, as for hard drives, desktop ones with 500 GB typically cost 70 bucks on tiger if you look. This leaves you with a 30 dollar buffer. I would personally spend the extr money and see if you can get a Seagate momentus XT 500 GB, it's an SSD hybrid drive, 4GB SSD, 500 GB Hard drive, and it will give you near SSD performance! It's about 100 bucks, take a look on tiger.