For a system using two Radeon HD 4850 graphics cards in 2-way CrossFire mode AMD specifies a minimum of an 550 Watt or greater power supply. The power supply should also have a maximum combined +12 Volt continuous current rating of 38 Amps or greater and have at least two 6-pin PCI Express supplementary power connectors.
Total Power Supply Wattage is NOT the crucial factor in power supply selection!!! Sufficient Total Combined Continuous Power/Current Available on the +12V Rail(s) rated at 45°C - 50°C ambient temperature, is the most critical factor.
You may find power supplies on the market that supply more than enough Wattage to run the system. However, some of them lack Sufficient Amperage capacity on the critical +12 Volt rail, which is necessary to properly power the critical components in the system (i.e. CPU and GPUs). This is the reason why graphics card manufacturers may overstate the power supply wattage, usually by at least 50 Watts, in an attempt to take into account some of those power supplies that have the weaker +12 Volt rail(s).
The Thermaltake Litepower 600W (W0355), with its maximum combined +12 Volt continuous current rating of 36.7 Amps and with one 6-pin and one (6+2)-pin PCI Express supplementary power connectors, is insufficient to power your system configuration with two Radeon HD 4850 in 2-way CrossFire mode.
Your PSU is not a 600 Watt PSU. It is a 500 Watt PSU. This is typical Thermaltake deceptive marketing practices.