Got my Sapphire 5830 Xtreme yesterday and I've been steadily ramping it up to its eventual failure @ 1020mhz core. I've found it's stable at 1010mhz core with memory underclocked to 300mhz. This is all at stock voltage, and frankly I couldn't be happier with what this card turned out to be capable of.
313.5Mh/s @ 1010mhz core 300mhz memory stock voltage -k phatk VECTORS BFI_INT AGGRESSION=12 FASTLOOP=false WORKSIZE=256 (in regards to 128 vs 256, for me at these settings the 256 worksize actually bumps my hash rate 5Mh/s)
What program are you using to get past the stock 900mhz? Are you on windows or linux?
I'm using Sapphire's own TRIXX software on Windows Vista x64. Nothing fancy at all, just download and install from their website. Only thing I'm limited to is stock voltage, for whatever reason it won't let me bump up the volts. If I set it to anything higher than stock 1163, once I hit reply it automatically reverts back to 1163, regardless of what I set the voltage to.