какой минимальной мощности должен быть блок питания, чтобы потянуть radeon 4850?

ATI Radeon™ HD 4850 Graphics System Requirements

PCI Express® based PC is required with one X16 lane graphics slot available on the motherboard
450 Watt or greater power supply with 75 Watt 6-pin PCI Express® power connector recommended (550 Watt and two 6-pin connectors for ATI CrossFireX™ technology in dual mode)
Certified power supplies are recommended. Refer to http://ati.amd.com/certifiedPSU for a list of Certified products
Minimum 1GB of system memory
Installation software requires CD-ROM drive
DVD playback requires DVD drive
Blu-ray™ playback requires Blu-ray drive
For a complete ATI CrossFireX™ system, a second ATI Radeon™ HD 4850 graphics card, an ATI CrossFireX Ready motherboard and one ATI CrossFireX Bridge Interconnect cable per graphics card (included) are required

ATI Radeon™ HD graphics chips have numerous features integrated into the processor itself (e.g., HDCP, HDMI, etc.). Third parties manufacturing products based on, or incorporating ATI Radeon HD graphics chips, may choose to enable some or all of these features. If a particular feature is important to you, please inquire of the manufacturer if a particular product supports this feature. In addition, some features or technologies may require you to purchase additional components in order to make full use of them (e.g. a Blu-Ray or HD-DVD drive, HDCP-ready monitor, etc.).

©2009 Advanced Micro Devices, Inc. AMD, the AMD Arrow logo, ATI, the ATI logo, Catalyst, CrossFireX, Radeon, and combinations thereof are trademarks of Advanced Micro Devices, Inc. Other names are for informational purposes only and may be trademarks of their respective owners.

Блок питания мощностью 450 Вт или 550 Вт для двух видеокарт в режиме CrossFire

500-600 Вт. Лучше брать с запасом

Кроме видеокарты у вас больше ни чего нет?
Параметры компа написали бы.

ват 400

незнаю

смотря на другие компоненты.. . а так в среднем вообще то около 400-450 Вт должно хватить...