Crash is likely due to instability.
This is the exact reason I recommend manually setting RAM timings and not using 'Auto"
When you are looking for a stable overclock, there are 3 key areas that can cause issues.
1. The CPU is unstable
2. The RAM is unstable
3. The NB is unstable
If you leave the RAM (Or any major setting for that matter) to "Auto" and crashes occur you need to guess where the problem is.
Setting things manually, and taking RAM out of the picture at the start, eliminates guess work as to the issue.
First thing you should do is Run MEMTEST to ensure the RAM itself is good.
(remember, just because MEMTEST passes, doesnt mean the system will be free of problems due to memory. It only means the RAM itself is good. You could still have NB issues for example under full load when OCd or a RAM failure)
Then, you manulally lower the RAM setting to just below what it is rated, say 667Mhz RAM @ 5-5-5-18 timings if its DDR2 800 CAS4 or 1066 CAS5 for example
Now you start OCing the CPU and get thimngs stable. If theres a crash or instability you know where to look.
Once you know what the CPU is capable of, then you can start tweaking the RAM.
The best stress test is Intel Burn test (MAX stress) at least 10 passes. This will give a fast idea if stable or not far faster than Prime. Within minutes rather than hours.
IntelBurnTest v2.3 download from Guru3D.com
Watch those temps!
If you lower RAM speed and test with CPU at a x9 multi, once you get an idea where it stays stable, you can lower multi to x6 and start working with RAM.
Once you have an idea of RAM speed, you can then up the multi and start working on them together with both those figures in mind (likely staying just below it)
Once all thats stable, then start working with Timings. (RAM timings should be the last adjustment)
Something else to keep in mind.
Hardware plays a huge role. If you have DDR2800 4-4-4-12 RAM for example, it may run well at stock speeds at those settings.
However, depending on the board and RAM itself, It may not be able to run at 800/CAS4 with higher FSB speeds. the board may not be able to handle it.