PLX is a PCIe lane switch (PCie lane endpoint manager), it does not have any more features than this or RAID related.
The card needs additional resources for additional features support, like RAID and boot, so thats what you need to look for.
That could be in the future …who knows… the next GEN of self-aware bios interface that would detect additional cards and their controllers providing/uncovering additional menu settings to control her functions…
But we’re not there yet… so answering your question, the bios can control the embedded controllers of her chipset, The Intel C600 AHCI/RAID and additional ones present in the mainboard, only.
The bios detects additional hw interfaces but cannot set/config her functions, this is usually done on the card when she possesses an Option Rom FW (OpROM) during the platform initialization.
So when looking for a card with RAID support, usually it comes with a OpRom of the controller used, ex.: Marvell, ASmedia, HighPoint, etc…