> An SB800 Southbridge would cost us four PCIe 2.0 lanes,
> but would also supply four PCie lanes.

According to the 'AMD SB810/850 Southbridge Databook' there're only two PCIe 2.0 lanes supplied, not four:

http://support.amd.com/us/Embedded_TechDocs/44758.pdf (page 12)

> Which configuration was it that you said we lost SATA controllers?

The one you liked most ;-)

https://morph.zone/modules/newbb_plus/viewtopic.php?topic_id=7183&forum=3&post_id=82381#82381

So let me summarize the two options:

1. without southbridge:
- SATA controllers on P5 must be enabled, thus x4 x4 x1 x1 PCIe config is best
- use one x1 with bridge chip to get legacy PCI slot
- for audio there're those options to select from:
-- sound card in legacy PCI slot
-- sound card in PCIe x1 slot
-- on-board audio chip on PCIe x1
-- nerdy: use P5's SPI bus with a CPLD acting as SPI-to-I²S bridge and connect an audio chip via this I²S ;-)
- one x4 for graphics card
- leaves one of the following PCI(e) connection configs for use at will: x4+x1 or x4+PCI or x4+x1+PCI

2. with SB800 southbridge:
- don't need P5's SATA controllers, thus x4 x4 x4 PCIe config can be used
- use one x4 to attach SB800
- legacy PCI provided by SB800
- audio provided by SB800
- additional two PCIe lanes provided by SB800, either two x1 or one x2
- one x4 for graphics card
- leaves one of the following PCI(e) connection configs for use at will: x4+x1+x1+PCI or x4+x2+PCI