Tom's Hardware took a look at the performance of the Radeon HD 2900 XT 512MB and GeForce 8800 GTS 512MB and checks how big the performance hit is if you save money by buying motherboards that only support PCI Express x4 or how much performance you're missing if you buy a 975 motherboard which features two x8 slots.
The results are pretty interesting, compared to x16 you get a performance hit of about 7-8% if your motherboard only supports x8 and if it can only do x4 your card will deliver 25-33% less performance.
They also ran some tests to check the performance difference between CrossFire cards that run on two PCI Express slots with eight lanes and a motherboard with two times sixteen lanes. In this case the performance drops about 12-15% if you're using a motherboard with two x8 slots.
Finally they also compared PCI Express 1.0a and 2.0 but in this case the difference was only marginal, the max improvement was 2.3%.