I have three 5830s and what I'm finding is that the 1st card (gpu0) mines the fastest, the second one about 20 Mhs less, and the third one about 20Mhs less than the second one. Right now they are at 300, 280 and 260.

System:
1000W PSU
Sempron 145
Asus m5a99x evo
4GB ram
Windows 7 + GUI miner using poclbm

Same situation with ubuntu