Woah 1ghz core clock speed??? Or memory because I think stock core is 625mhz and I have read reviews by people who had it not function with less than 100mhz increase.
The stock core is 725Mhz.
1Ghz applies for the core clock, though you're actual results will most likely be around 900Mhz as individual chips have different capabilities and cooling is different for every case.
The 5850 is a nice OCer and the DirectCU has software voltage control which pushes the limits much further. Ironically, the Memory clock should not OC nearly as high, 1300Mhz is around the max.