Hey, that's great! Did you disable things one at a time? That way you can tell what was causing the problem. There is a thread going on wether or not to disable EIST and C1E during/after OC-ing here: http://forumz.tomshardware.com/hardware/OC-speedstep-ftopict241791.html
The most sensible (to me anyway) thing I've seen in that thread is OC first, then try enabling EIST/C1E and see if it stays stable. If so... great, if not... disable it again.
As for how high you can OC on stock cooling depends on temps mostly. I like things to run cool so my upper limits are 50 C load. Some go up to 60/65 C load but thats too hot for my taste.
Oc to your desired setting-> test temps while running idle-> test temps while stress testing with orthos/prime95 to see if it exceeds you're comfort zone/dangerous temps -> stress/stability test for a few hours. If it passes everything you can throw at it (incl. games and benchmarks) and temps are safe, you have a succesfull OC.