actually for some cool server performance stuff a raspberry pi is not really great, especially since he is missing at least some sata interface. Beagleboard is better but still misses sata
The new Beaglebone has eSATA, indeed it will not be that great for high-performance SSDs or many server applications but it could be perfectly fine for a PC.
Dual gigabit Ethernet ports and USB 3 round out the impressive X-15's impressive features. It's not available yet but should be by the end of the month or early July based on the wiki. If it runs OpenBSD I might choose it over Soekris.
Ah yes I forgot about the dual ethernet ports, it might be really nice paired with a good ethernet switch as a router system!