the dillema is if you go higher in CPU speed you probably have to drop the FSB and increase the multiplier to keep your RAM working ok. Therefore, even if you're able to hit 2.5GHz at 250x10, that will be a 4% increase CPU speed but your memory FSB will drop 6%. Overall it will still be faster in most applications but only 2-3%, at the cost of having to burn your CPU hotter with more voltage
So all in all with the 3500, 267x9 is actually a very optimal overclock.