Those are my stable clocks on this Zalman VF700-Cu. I'm sure I could go higher, but there really is no need at the moment. Note that these cards draw an outstanding amount of power. Single rail (+12v) PSU's are preferred with X1K cards, in most cases. If you have a dual rail PSU and your card is unstable even at stock, investigate a bit. Check both of your +12v RAILS with a Digital Multi-Meter. You can get a DMM at your local Radio Shack for $19.99. If your rails sag a noticeable amount (under 11.80v) under load, you either need to adjust your rails via pots if your unit has them, or you will need a new PSU.
If you're lucky enough to have a Tagan PSU with a switch on the back, to select either a single 12v rail or dual 12v rails, slide it over to a single 12v rail.
Here are my voltages (idle, 0.03v less compared to my DMM)..
Idle:

Load:

(Click to view full image)
Per normal, always take caution when overclocking. Please take note, that these cards have been known to lock up and send the monitor into standby mode when the core is pushed too high with too little voltage. So, if you're OC'ing and your computer freezes, then your monitor goes into standby mode... do not fret. Just press the reset button on your case and you'll be back up as soon as your computer boots up again.
To have ATi Tool apply these settings at startup, please read the next page.