Unless you're doing serious gaming, you don't need SLI or Crossfire (multiple video cards).
For what you've listed as your primary uses, I'd recommend any quad-core processor (Intel Core i5-750 would provide great performance for about $200, but AMD Athlon 2 X4 640 provides about 80% of the performance for about 60% of the cost). Any motherboard compatible with your chosen CPU would work; good brands include Asus, Gigabyte, and MSI. The minimum amount of RAM you want is 4GB, and if you can afford it, get 8GB DDR3.
An AMD Radeon 5750 should be ample graphics horsepower for your needs, and storage can be handled with a 1TB HDD (Western Digital and Seagate are the best).
You'll also want a BD-ROM drive for watching/ripping Blue-ray movies (it can also handle DVD's). I would also recommend a dedicated DVD-RW drive.
Get a good, name brand power supply (Coolmax, Thermaltake, Coolermaster, etc.). 500W should be more than enough power. Also get a good mid-tower ATX case. I like the Cooler Master Centurion series, but there are plenty of good cases on the market between $50 & $100.
Newegg.com is my preferred shopping site. Their prices are competitive and the user reviews give you a good idea on what's good and what's trash.