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)