Use either ati tool or ati tray tools. I use the latter, because it has all kinds of tweaks - it's like having the CCC only without all the bloat. So, basically find the max that it tells you, and then back down by like 10mhz. Also download and run rthdribl, it's very sensitive to artifacts, and you can see the artifacts if you OC too far.
I used to have a r360 9800p with the hynix mem, and while the gpu and the mem could be OC'd pretty far (I ran mine at 440/375) you will need an aftermarket cooler to get any significant OC and keep the card from overheating. The mem especially gets really hot even at stock speeds, and it wont OC much without ramsinks.