RISC is a type of instruction set used in CPU's think IBM's Power line, like the Power6's used in their P Series - and the SPARC line of cpu's used in Sun systems, old Mac's from the 6100's all the way up to the last G5's.
Sun is supposed to be sexy, I have a few Ultra Sparc boxes here, not the newest but they sure do hold their own. Only gripe is the Common Desktop Environment. I hate it.
Unix is pretty badass. We have a few flavors here, IBM's AIX ( our our P Series systems) and HP-UX on our Superdome mainframes. AIX is slightly more badass than HP-UX because of smitty. Smitty (or smit for the old people) makes life much easier when setting up a new system.
Now, why did they leave out OS2 and put in 'GNU'?
If you were wondering, why yes I work for a datacenter.