Indeed RAM is the easiest thing to kill overclocking.
So it all depends what RAM you are running (speed its rated at) and what FSB you are running at.
This is why many overclockers go with DDR2 1006 or higher RAM for an Intel system they will be overclocking.
If you have DDR2 800, I wouldnt push it to far beyond that.
For 1066 RAM, you have much more room to play with. You can run at 1:1 or even a bit more.
Such as a strap of 333/800 for example.
You may not alway run at its rated speed, but once you reach the desired CPU OC, you can then tighten timings..
General rule of thumb, try to OC without actually OCing the RAM. In other words, if your going to OC, get RAM rated at a faster speed than you really need.
For most socket 775 Intel systems, 1066 is about right.
But even with DDR2 800 you can get respectable OCs at a 1:1