Here is a simple comparison chart for youhttp://www.hardwarecanucks.com/forum/hardware-canucks-reviews/31444-amd-s-890fx-chipset-evolution-am3-platform-2.html
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.