Windows 7 BSODs

I bought a Gaming PC about 6 months ago, it was fine for 2 weeks or so... then started getting BSOD's randomly when using the computer. Typical ones are:

Fault in Non Paged Area
Memory Management
Bad Pool Header
Bad Pool Caller

When I started getting them I had these specs:

AMD Athlon II x2 250
Asrock N68-S3 UCC
8GB DDR3 RAM
ATI HD 5450 1GB GPU
A standard 500W PSU
Hitachi 1TB HDD
Windows 7 Home Premium 64 bit

I have no Idea what the problem is, I bought a brand new copy of Windows (Windows 7 Professional) and even added new hardware, I now have these specs:

AMD Athlon II x2 250
Asrock N68-S3 UCC
8GB DDR3 RAM
Radeon HD 6850 1GB GPU
Antec 620W Gamers Editon PSU
Sandisk 500GB Primary HDD
Hitachi 1TB Secondary HDD
Windows 7 Professional 64 bit

I thought It might have had something to do with the HDD so I bought a new one, turned out it wasn't the problem. I upgraded my Graphics Card also which required a much better power supply. It doesn't happen all the time but it's very annoying. I have updated all drivers on my PC and I am still getting the BSOD's... I really don't know where to go from here... I'm curious to whether it's the RAM or not.

If anyone could help me out I would greatly appreciate it. Thank You.
 
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