[jpstewart]

Why won't the Fuel DVD work in the Indigo?  I know I used a Pioneer slot-load DVD-ROM drive to install IRIX on my R4K Indigo.

For IRIX you have a wide range of options from 4.0.x all the way up to 6.5.22 on the R4K.  If you max out the RAM (192MB on R4K, IIRC) IRIX 6.5.22 is useable.  Not fast, but useable.  If you have less RAM, use something older for sure.  I run 4.0.5F on mine, and that feels super fast and responsive.  But not a lot of software is available and building open source stuff on an OS that old is a challenge these days.  (But I like that challenge, personally.  Maybe you will, too.)  If you're already leaning towards IRIX 5.3, that's probably a good middle ground between the extremes:  it'll let you run the "older apps" you mention without feeling too archaic.