Note, This page does _not_ cover the x86-based Qube 3, Raq 3 and later Cobalt servers. I've got some resources on my wiki here.
All the Cobalt servers, including the original Qube right up to the Qube 2/Raq 2 use the same Quantum Effect Design RM5231 CPU running at upto 250MHz. This processor is a MIPS IV architecture with minor extensions. At the moment, Linux/MIPS will run in 64-bit mode on these machines, but support is very experimental at the moment, and therefore is not recommended for production use. Running a 32-bit kernel however is quite stable.
These boxes all have two 72-PIN EDO slots on the same board as the CPU. The boxes came with standard amounts between 32 and 64MB RAM, however can take up to 128MB without any problems. (They might even take more than this, but I can't source 128MB SIMMs in order to test this)
These boxes are completely headless, using a serial console that runs at 115200 bps (8-bits, no parity, 1 stop bit). To enable this during the startup, hold in the "password reset" button whilst powering the unit on. When "Console ON" appears on the LCD panel, power the box off, then back on again, from there you should see the Cobalt firmware (or CoLo) starting up.
The original Qube, as well as the Qube2, had a single ATA66 IDE socket on the main board. The Raq2 has an onboard LSI-Logic (at least in this photo that's what it looks like) controller which hooks to a 68-pin SCSI port at the back. The Qube2 can happily accept any PCI SCSI card that will physically fit inside the case (I'm happily using a Adaptec AHA-2940AU PCI SCSI card without any problems).
Note that the Cobalt firmware only recognises the Master IDE HDD plugged into the onboard socket.
All of the Cobalt servers are equipped with two 10/100Mbit ethernet interfaces. These interfaces are a very close relative to the DEC Tulip series of network cards. Again, if your server has a PCI slot you can install a PCI network card into that slot to use it with Linux. Note that you can only netboot with the onboard Primary (eth0) interface.
The stock firmware on the Cobalt servers is basic to say the least. It supports netbooting using DHCP & NFS, as well as booting off the hard drive. The firmware expects to see a kernel image called 'vmlinux.gz' on the root directory of the first partition on the first drive. It can only read EXT2 revision 0 partitions (create them using mke2fs -r 0 /dev/BLAH) and cannot load kernels of over 675kB in size.
Peter Horton is the current maintainer of the CoLo boot loader. CoLo, unlike the original bootloader, has no limitation on the size of kernel to load, and also features support for initial ramdisks, EXT2 and EXT3 support, as well as loading kernels over NFS and TFTP. It is configured through the use of scripting, with the capability to chain scripts together, ask questions on the LCD panel and perform various tasks. CoLo can be found here: http://www.colonel-panic.org/cobalt-mips/