I'm running 2 6990s (both with the OC switch on the overclocked position, but no further OCing) on a single-core Sempron 140. With cgminer, one card gets the expected performance, but the other one is incredibly low (40-50Mhash per GPU). I'm using an intensity of 9 as suggested in the original post for 6990s.

This is the exit output:

 GPU 0: [395.7 Mh/s] [Q:91  A:85  R:3  HW:0  E:93%  U:5.55/m]
 GPU 1: [395.6 Mh/s] [Q:92  A:96  R:1  HW:0  E:104%  U:6.20/m]
 GPU 2: [39.4 Mh/s] [Q:12  A:5  R:0  HW:0  E:42%  U:3.13/m]
 GPU 3: [46.4 Mh/s] [Q:13  A:10  R:0  HW:0  E:83%  U:5.95/m]