Hello i`m new in mining and having problems to make my machine work.

My mother is a Gigabyte ga-ma790gp-Ds4h, it has 2 pci-ex.
I have one XFX 6850 and one XFX 4850
My power supply is a 600w one ( 22 amp ech line )

The 6850 mines ok, when i try to make the 4850 start mining system crashes.

If i connect just the 4850 works ok.

I used Catalyst 11.3 and 11.4 both with this problem.

What do you think the my problem is?