Everything's highly dependent on how good of an airflow you have in your case, whether you have any "hotspots" where hot air gets trapped, whether your air circulation from front -> back is not blocked by bad wiring, etc .... lots of factors.
You have one of the better MBs for the job, so in terms of stability you probably don't have anything to worry about it. I would say that a 333Mhz FSB (likely at stock Vcore and northbridge voltages) is a fairly safe OC level for you, using the stock HSF.
That would give you a 333/266*2.13 = 2.66Ghz CPU speed, or in other words an E6700 (minus the 4Mb cache size ... but that doesn't seem to have a hugh performance impact from the benchmarks I've seen).
I've only OC'd core2s for 2 friends of mine, and personally I do not have a Core2 system. However, I'm confident that if you stick at those levels, you have nothing to worry about (on stock cooling). You can likely go much higher on stock, but that would involve higher voltages -> higher temp and higher wattage, might compromise stability unless you get some better cooling. Good case airflow is also essential to keep the MB cool.
Cheers and GL
edit : In your situation, considering you ahve 800Mhz ram, the ideal (in my opinion) setup would be to have your FSB at 400Mhz (effectively 1600), which would allow you to run your ram at "full speed". So far the word on the street is that a Core2 performs best when the memory speed is synced (1:1 or 1:2 of FSB), not when there's some weird 9:13 divider setting. More so than with older pentium Ds.
Running your FSB at 400Mhz, however, would certainly require some form of decent aftermarket HSF. Scythe, Thermaltake, Tuniq all make excellent coolers, priced around 50$ (give or take 10$). Considering the performance you gain from running your CPU at higher speeds, as well as the fact that a good aftermarket cooler is less noisy (even if it's much more peformant) than a stock Intel cooler at full speed, the tradeoff is usually worth it. Will require some good research on your end tho