Search the board for a Compaq partnumber like 123456-001 and feed this number to the hp partsurfer tool at www.partsurfer.hp.com would be the easiest way.
I don´t recognize the board myself, but it may be something from the low end Proliant ML line, like the ML350 G2. From what I found on the web the ML350 G2 comes closest, so probably its a different revision of the board.

The slot next to the CPU sockets is for a VRM module to feed the second CPU. The Slot looking like AGB is probably something proprietary, AGP slots start at a greater distance from the back of the board and AGPpro extends to the other direction as well.

I can think about 4 possible uses of that brown port. A slot for a "memory riser" to further expand system memory (likely, they have slots that look suspiciously like that), A board to board interconnect for additional CPUs (unlikely, too far from the CPUs) A slot for a PCI riser card (unlikely, never seen riser cards for boards with PCI-X) or a port for a proprietary storage controller ( no idea how likely that is)

Thanks all for thinking along. It's apparantly the mainboard of a Proliant ML330 (2nd Generation).

The brown slot is referred to in the manual as "Server Feature Board slot (slot 6)".

This slot provides the SCSI or ATA interface. I've included a page from the manual:

Right, should have known that. These Compaq servers have the basic features integrated on a single card.
These usually include VGA, ethernet and SCSI or IDE. This keeps the PCI slots free for other add on cards. Interesting fact is the battery on the add on card, so the add on boards seems to contain at least parts of the RTC. Not sure if you will be able to run the board without the add on card, but they usually show up on ebay frequently and sell for small coin due to the limited number of posible users. You just have to wait for some time to find a cheap one, because the professional sellers usually ask something based on the original retail price for them.