Personally, I've gotten mine up to 1.15GHZ stably with all voltages under what Titan's XLV settings were:
0:22,90 125:22,180 250:22,180 500:30,360 550:32,400 600:34,430 700:39,430 750:41,430 805:45,430 900:50,500 950:54,500 1000:58,500 1100:67,520 1150:71,520

With those settings, I can run it under full load indefinately at 1GHZ, though it gets warm in my hand. 1.1GHZ is good for about 5 minutes before it gets warm enough that I stop testing; probably less than 50C core though. 1.15GHZ, about 3-4 minutes.

Note that those are under full load - If I planned to watch movies on my N900, something that needs as much processing power as it can get, I'd go with 900-1ghz max. If I was looking for quick response, say in webpages, but not something that will keep the processor loaded, 1.15ghz is probably fine.

Now, those are with my voltages. If your settings are higher, you'd probably want to clock it lower so you don't overheat it, or run it less time under load.