manufaturers usually leave a 10% margin for safety precaution. If you overclock it much, use a heatsink on your memory and a good fan on your GPU.
from your OC software, start increasing it by small steps, something like 5 or 10 MHz and then run a program like 3Dmark and check display for any corruption (white speck or other things that shouldn't be there). When you start seeing them, revert to the frequency where you didn't see them.
Do this sequentially for your GPU and then your memory. Note that as with everything else, the max OC frequency will vary from card to card and may shorten your card's life, specially if you do not cool it properly.
This technique takes some time but it is one safe way to find a stable OC frequency.
If you do a google search, you'll find several pages which will give you hints and OC results on this card.
My gaming machine:
P4 2.8 GHz
2 X 512 MHz PC3200 in dual DDR mode
Intel D865PERL MOBO
integrated 6.1 MAX audio
Logitech duo MX mouse and KB
ATI RADEON 9800 PRO