Just did a small amount of testing to see what happens where (mainly with refresh rate and VSync). 
Here are some figures: 
1400 x 1050 x 32 bit - Water 2, Effects 1, 2xAA, 16xAF - V-Sync Off - CrossFire 
2008-02-09 21:54:14 - il2fb 
Frames: 5269 - Time: 120000ms - Avg: 43.908 - Min: 23 - Max: 121 
1400 x 1050 x 32 bit - Water 2, Effects 1, 2xAA, 16xAF - V-Sync On - Refresh Rate 60Hz - CrossFire (x8 x8) 
2008-02-09 22:16:07 - il2fb 
Frames: 4128 - Time: 120000ms - Avg: 34.400 - Min: 18 - Max: 62 
1400 x 1050 x 32 bit - Water 2, Effects 1, 2xAA, 16xAF - V-Sync On - Refresh Rate 60Hz - Single Card 
2008-02-09 22:20:46 - il2fb 
Frames: 3498 - Time: 120000ms - Avg: 29.150 - Min: 8 - Max: 62 
Notice the minimum FPS and total Frames rendered drops significantly.