AMD's replacement for 790FX and the SB750 Southbridge won't arrive until Q4 2009. Previously many expected to see this chipset in very late Q2, roughly Computex time, but according to the latest info this simply won’t happen as soon as many expected.
The chipset is AM3 only, and AMD wants to get rid of AM2+ legacy and infrastructure and continue with DDR3 platform. The new RD890 chipset supports HT3, 2x16PCIe or 4x8 with second generation PCIe , as well as enhanced Quad Crossfire X 4x8 support.
The SB850 Southbridge has all the features of SB750 Southbridge as well as Gigabit MAC, AHCI 1.2, new clock gen as well as second generation PCIe 4X.
Until Q4 2009, AMD 790FX or AMD 790GX integrated chipsets will remain the only two for the new Phenom AM3 based CPUs, at least until the RD890D comes out.