Unfortunately he asked about Crossfire and not SLI.
Here is a good review that shows performance with a single Radeon 5870:
AMD Radeon HD 5870 PCI-Express Scaling Review - Page 25/26 | techPowerUp
The performance difference with a single card is around 2%...so not too much. When you add in a second card I'm sure the difference is larger since the cards use the PCIe bus to communicate to eachother in addition to the rest of the system.
In real world gaming you probably wouldn't noticed the difference. If your into benchmarking, the difference is huge.