I realized that many people make a mistake overclicking their cards not properly or setting up guiminers wrong. What I'm about to say was checked on 5870, 5850 and 5830. Every time there was a few % more mh/s when I used those settings.
Try this settings:
1. Core Clock - that's up to You.
2. Memory clock - 340MHz ( not less not more )
3. Extra flags: -v -w256
4. Assign all cards to one core.
5. No cross fire
5870,5850, 5830 seem to work as fast as they can on memory down clocked to 340Mhz - much faster. Anything below or abow is slower.
XFX Hd 5870.
Core - 950
Memory - 340
Voltage - stock
Extra flags: -v -w256
CPU - Phenom x2 550 down clocked to 800MHz
GUIMiner - 433-438Mh/s each
XFX HD 5850
865/300 = 350m/h
865/340 = 352.30m/h