I bought and returned a 1U unit from Supermicro based on that same motherboard (X7SPA) ... this unit specifically: http://www.supermicro.com/products/system/1U/5015/SYS-5015A-PHF.cfm

Technically, everything was detected just fine. The NICs were supported w/ ClearOS 5.1. The main issues I had was :

a) I couldn't get the file transfer rate above ~15Mbps over a 1Gbps network (I can get ~45 to 50 Mbps on my other machine). I'm assuming this was the particular NIC controller on the mobo ... which is Intel 82574L (but not positive).

b) Supermicro does not have a riser that will work w/ the PCI express slot. Therefore the slot is useless unless you go for one of the aftermarket "flex" riser routes.

I bought it to power a home built NAS with an external eSATA RAID array using a PCI-E port multiplier card ... so I just wasn't satisfied w/ 15Mbps.

That mobo / server would work great as a firewall gateway from what I could tell though. Just wanted to give my experience with it ...

I just ordered parts for a similar system.

I choose the Intel D510MO (which oddly enough, has one Realtek NIC)
I'll use the PCI slot to add a low-profile Intel Pro dual NIC card (Ebay seems to be the only place to buy at a reasonable price)

I considered the Supermicro X7SPA-H, but I think I remember seeing some reviews saying that the NICs were not currently recognized by some firewall/linux OSs (Clear was not mentioned)
It also has a PCI-Express slot rather than PCI. This was a minus for me. It also uses SODIMM memory (but I don't know if this really costs more)

Other parts;
case: Antec ISK 300-65 (Good review on silentpcreview.com)
hd: Seagate Momentus 7200.4 ST9320423AS 320GB 7200 RPM 2.5" SATA (good review on silentpcreview.com)
ram: Kingston ValueRAM 2GB (2 x 1GB) 240-Pin DDR2 SDRAM DDR2 667 (For me, it's Kingston or Crucial)

I'm not putting in a DVD drive, so I didn't worry about the extra cost for "slim" DVD.

About $270 total. I'm hoping/expecting it to run in under 25 watts.

Hope this helps.

On a default, newly installed 5.2 system with all the modules post install and post-wizard.

[root@system ~]# df
Filesystem 1K-blocks Used Available Use% Mounted on
/dev/sda3 19210732 1882944 16336176 11% /
/dev/sda1 77749 11484 62251 16% /boot
tmpfs 257640 0 257640 0% /dev/shm

(parted) select /dev/sda
Using /dev/sda
(parted) print

Model: VMware, VMware Virtual S (scsi)
Disk /dev/sda: 21.5GB
Sector size (logical/physical): 512B/512B
Partition Table: msdos

Number Start End Size Type File system Flags
1 32.3kB 82.3MB 82.2MB primary ext3 boot
2 82.3MB 1160MB 1078MB primary linux-swap
3 1160MB 21.5GB 20.3GB primary ext3

You should NOT run swap on your compact flash or other device like that because your changes will burn out those sectors quite quickly.

Use the custom disk partitioning and do NOT provision any swap. Afterwards, if you must have swap, you can run it on ram.

If you need performance, SSD is the way to go and the prices are dropping.

Outside of that, your only real danger is user data, and logs. This can be place on RAM as well or even on external USB. You can allocate the directories to these other devices even by way of using a bind mount.

Also, note. write speeds on compact flash is bad so your system will not be as fast but it will work.