Check the stability with prime95/orthos and rthdribl (Real-Time High Dynamic Range Image-Based Lighting)
http://www.daionet.gr.jp/~masa/rthdribl/
These will max out your cpu as well as gfx card, in other words max out your system recources.
The machine would crawl in terms of performance but if it is still running after say 30 minutes with no error and also with cpu and gfx temps under limit, then you can say that the overclock is stable.