Video Card artifacts, Memory error, BSOD = all in one

Hello,
I bought used parts.


Intel Core i5-760
Asus P7P55D
OCZ Gold 8GB (4x2gb) DDR3 PC3-10666 CL9
Saphire Radeon HD 6850
Enermax Liberty 400Watt PSU ELT400AWT
For now I use HDMI cable not DVI to connect to a monitor.


When I first turned on computer first thing I saw were artifacts.
http://oi45.tinypic.com/fep6vr.jpg
http://oi49.tinypic.com/14mue5s.jpg

I thought video card is broken, so I need to change it, but that's not only problem I had.
I couldn't even install Windows 7 32 or 64bit. I got memory errors all the time.
So I removed RAMs one by one.
What I noticed with RAMs are -
1) with 4 RAM modules I couldn't install windows I got memory errors
2) With one RAM module computer didn't even start, just black screen and no beep sound at begin.
3) also computer didn't started when I put two modules in slot 1 and 3.
4) I could start computer and managed to install windows with two modules in slots 2 and 4.
5) I could start windows (already installed) with all 4 modules. And it showed 8Gb RAM. But time by time I got Memory

errors
6) Windows Memory diagnostic tool didn't show any error.

I installed ATI drivers and artifacts became even worse.
http://oi49.tinypic.com/2dha2o0.jpg

Catalyst stops to work constantly, with all four RAMs windows explorer stops working and restarts all the time.

Sometimes I get blue screen.


Bug Check String MEMORY_MANAGEMENT
Bug Check Code 0x0000001a
Parameter 1 00000000`00041790
Parameter 2 fffffa80`0635c9a0
Parameter 3 00000000`0000ffff
Parameter 4 00000000`00000000
Caused By Driver ntoskrnl.exe
Caused By Address ntoskrnl.exe+71f00
File Description NT Kernel & System
Product Name Microsoft® Windows® Operating System
Company Microsoft Corporation
File Version 6.1.7600.16385 (win7_rtm.090713-1255)
Processor x64
Crash Address ntoskrnl.exe+71f00
Full Path 1 C:\Windows\Minidump\101912-14570-01.dmp
Processors Count 4
Major Version 15
Minor Version 7600
Dump File 272,408




What may cause so many problems at once?
It seems that one or even two parts are broken, most likely broken is/are -
*PSU
*Motherboard
*Video Card
*RAMs