OK. Just ran Heaven for 1/2h with the VRAM at 1400 and no problem. Not gonna try higher. That seems like a nice number.
Final results I'm settling at, if someone wants to update the o/c database:
HIS 7850 IceQ X Turbo
Core: 1125 MHz
VRAM: 1400 MHz
Voltage: 1.210 (default)
Ambient temp: 27C
Fan speed: user-defined, constant 40%
Full load (Unigine Heaven) temp: 66C
My little (9.4") card should be performing close to a default 7870 in Skyrim (AnandTech GPU bench shows a difference of 11-12% at 1920x1200) at those settings, so I'm happy.
Edit: card can play Skyrim & run the Heaven video many times without problem at 1150 core but the benchmark crashed near the end just now, so I downthrottled the core to 1125 and it benches OK now. Changed the results above accordingly.
Will run the benchmark a few more times to make sure it's stable.
Edit 2: Just ran 15 instances of the Heaven benchmark at 1125 without problems.
Now that I've tested its stable limit, the card is back to its default 1000/1200 as I don't need the extra speed for Skyrim.