PCIe x16 slot speeds?

[Zemfik]

Can someone explain to me what it means for dual PCIe x16 slots to have one at x16 and another at x4 (or x8, or whatever)? What exactly does this affect? Is it much worse than having them run both at x16? Is the X38 chipset the only one that supports both at x16?

Thanks.

[oblivious_maximus]

It's all about bandwidth. There's some good info at wikipedia http://en.wikipedia.org/wiki/Pci-e

basically 1 'lane' = 250 MB/s, 32 lanes (the maximum 1 slot can have with pci-e 1.0 apparently) = 8GB/s
Putting this into perspective, a single lane has nearly twice the data rate of normal PCI, a four-lane slot has a comparable data rate to the fastest version of PCI-X 1.0, and an eight-lane slot has a data rate comparable to the fastest version of AGP.

Some parts don't need more than lane of bandwidth (a PATA controller for example) while other parts like video cards or raid controllers or 10Gb ethernet adapters can and do use a lot more bandwidth.

the new AMD 790FX chipset (as another pci-e 2.0 chipset like the X38) also supports multiple slots at 16x. I don't know the specifics of what's required to have 2 or more 16-lane slots(likely something to do with the total available bandwidth and how many other slots there are), but I understand that there have been(and still are I assume) motherboards on sale with 2 and more 16-lane slots before X38 and 790FX were available.

edit: 1st reply was crappy. this is less so.