You will notice that the 790fx and 890fx both use an older manufacturing process and may indeed actually be the same chip. Still, the 850SB provides faster SATA ports and 2 more USB ports. The biggest addition to me is RAID 5, but higher end 790fx chips often include that as well.
If you're going to just get a quad core chip, an AM3 790FX board is probably the way to go. Only pay more for the 890FX board if you're going with a 6-core chip. Of course, make sure that the shipping BIOS supports it.
You also get USB 3.0 Support the next step in USB evolution and SATA 6GB/s doubling the speed of the previous 3GB/s SATA seeds (although it may take some time before these new technologies reach their full potential).
I'm going to use a 6 core processor. One thing I've never learned over the years is how to flash a bios (apart from inside windows). I'm going to have to learn this fast as I imagine I won't get 6 core support out the box (mainly because I a cynical fool). In fact will I be able to flash a mother bios with a processor that isn't supported by its firmware? Time to do some googling. Thanks for your help.