BTW, What's the R12ka optimized port status?
As far as I know Linux will run, but crash after 5min-hour because of the R10K cache issues. Octanes have hardware to prevent corruption and BSD uses a whole other set of DMA routines which is really what prevents Linux from being stable. NetBSD and OpenBSD should have R10K and R12K O2 ports, but the work done for those OSes don't directly apply to Linux. The best info. on the subject is here:
http://www.total-knowledge.com/progs/mi ... ssue.shtml
The same site has some patches, but work is slow...
As far as Linux/ Irix on a single drive, that's a walk in the park. You just need to use arcload (or the more complex arcboot) and supply the right partion numbers and kernel names. Tell it to boot one by default and if you want another OS, interrupt the boot sequence and type boot Linux or boot Irix (or whatever you name them). I do this all the time to control what kernel I load and whether to load my Debian or Gentoo partitions. I actually haven't tried Linux/ Irix dual boot since I have an R12K O2 that runs Irix and I use my 600 Mhz one purely for Linux, however, I know its possible. You may need a larger hard drive (or to resize your volume header since its advantageous to put both kernels, arcload, and config files there) to do this. I'd highly suggest trying the Gentoo SGI Live CD first though.
If you want to try with arcload, I can help. Arcboot is a whole other issue. Gentoo uses this instead (by default), and it doesn't seem to properly overwrite the OSLoadPartition PROM setting so you either need to change PROM variables before booting (annoying) or do some major trickery to get Irix to load. See this (rather confusing) thread:
http://forums.gentoo.org/viewtopic-t-54 ... +boot.html
As far as power, I'm okay since my 600 Mhz processor draws < 3 Watts (it probably could run without a heat sink). Trying to add a modern video card on an R12K probably isn't the best idea, but for heat issues rather than power. Open the case of your running R12K and feel the giant heatsink after 30 minutes, rather warm is an understatement.
The fastest PCI ATI card you can get (at this time) is an X1650 and still only draws around 30 Watts full load, which is the max a standards compliant PCI slot can provide. I know the BFG PCI says 250 Watt, but I think this is more of a cya number since most power supplies inflate their wattage. Also, the PCI board doesn't have a power connector so it must be under 30 watt, like the X1650. As far as the numbers on the box go, I've run P4s off high quality 150 Watt power supplies when the box said 300 minimum (and drew a ton of flak for that) but using a Kill-A-Watt measured the real peak load of that system to be closer to 135 Watts. Since an O2 can also accept a secondary video card and video capture board, I'm fairly sure power supply isn't an issue unless you really overdue it. Your PCI extender may need to tap into the power supply directly if you plan on using multiple bus-powered USB devices, but I'm sure its doable, even on an R12K.
Finally. you won't think this from the monster heatsink and how hot it gets, but the R12K @ 400 Mhz is only a 20 Watt processor (according to wikipedia). However, I'm fairly sure about this number since Hennessy and Patterson mention the R14K @ 600 is 17 Watt and that number is also in the Wikipedia chart. That means 150 watts for all the other components in an O2.
Anyway... sorry for being long.