Finally switched to Nehalem system and I've seen the last of my full of problems S2E, C1 YORKFIELD, CORSAIR 4GB 1600C7 system. After 1,5 years of troubles these are my points of view:

1) C1 Yorkfield sucks. I ended up running my Q9550 below the 4 GHz mark because it couldn't do it with a proper VCORE. It's wasn't a board limitation.

2) Overclocked system behaviour:

- PRIME95 passes OK
- Memtest passes OK
- Temps OK
- PSU voltage readings OK, and:
- Random BSODs, mainly when accessing the hard drives. Biggest stress seems to be full formatting a hard drive.

The solution that worked for me: Unlinked the FSB from the memory and set the timings slightly below its rated specs at DDR3 1600 C7.
I never suspected that with PRIME95 and MEMTEST givving me no errors, my problems would be memory related, but they were.
Even the simplest overclock of setting FSB to 400 and the memory linked at 1600 was givving me BSODs. Only unlinked was trouble free for me.