Started off with

Abit IP35-E
4x1gb (2x ballistix 6400, 2x ocz gold 6400)
Silverstone ST50EF+
4x hdd

Dropped in a visiontek 4850, and then I started getting random blue screens with atikmdag.sys as the problem. Replace the board, then the cpu, then remove some ram, so I'm left with

Asus P5Q Deluxe
2x1gb OCZ Gold 6400
Silverstone ST50EF+
4x hdd

Works fine for a while, now I've started getting ntfs.sys bluescreens, random dwm crashes. Run checkdisk, no problems found at all. Ramtest checks out fine, as does Furmark, and OCCT.

psu multimeters out properly, and kill-a-watts in at under 400w.

Any ideas?