For your needs/budget I would recommend:
CASE: Antec 900 - $99.99
or CM 690 - $69.99
PSU: OCZ 1000W ProXStream - $59.99
The above is a recertified model that is on extreme
sale 70% off, if they run out of stock before you order, any 600W or greater OCZ or CORSAIR unit will suffice.
CPU: Phenom II X4 955 black edition - $175.99
MOBO: Any motherboard compatible with the P2 X4 955 that has other options you want.
VIDEOCARD: I would recommend a HD 4870.
RAM: 4GB of DDR2-800 or DDR3-1333 or DDR3-1600
HARD DRIVE: A 7200RPM hard drive with as much space as you need, or 2 of them in a RAID 0 array.
OS: Windows 7 Home Premium 64-bit for System Builders - $106.99
Do you need an operating system, monitor, peripherals?
Will you be doing any encoding, photo/video editing?