I just bought an LSI SAS 9211-8i HBA(Host bus adapter) to use with my new hardware. I realized after looking at the card that it required an x8 slot and my new mobo only x16 available. I keep on reading that PCI-E specs are backwards compatable however when I try to install Windows 7 on it it does not find the RAID Controller and now I am really confused. Can anyone help with this one?
Yes, in the general case you can install PCI-E cards in any PCI-E slot that will physically accept the card. If there are fewer PCI-E lanes available than the card requires, it should run a little slower but still run. If there are more lanes, it will just use what it needs.
PCI-E 1 and PCI-E 2 should not be a problem either.
Regarding Windows 7 not finding the RAID controller, I do of course assume that you're using the proper driver for the RAID card? And that if you're installing the OS onto a RAID volume, that you're loading the drivers during installation...
If all this is the case, then my guess is that you're using a consumer class desktop motherboard. Some desktop boards have PCI-E x16 slot which will only work with graphics cards. This goes against the intentions of PCI-E, but some manufacturers don't bother to do anything about this, since desktop machines almost always use the fastest PCI-E slots for graphics. Try contacting your motherboard maker / PC suppliers support..
Yes, you can plug x8 cards into x16 slots. It's more likely that your problem is that you've got a PCIe Gen2 (version 2.0) card in a motherboard which doesn't know how to deal with that. Try a BIOS upgrade.