I'd definately say the SGI. Used one of these at work a year ago, and it just plain kicked ASS. I've used other flat panels, and they just don't compare. For me gaming isn't as much of an issue as good video playback and having a *hitload of xterms and mozilla windows open at once, so perhaps your needs may differ. I've heard glowing reveiws of this thing running FPS games though, due to its excellent refresh and dot pitch (although obviously they won't fill the whole screen, few games allow you to play at 1600X1200.
And yes, technically it is "old" technology in terms of when it was introduced, but keep in mind that it was orginally only for the most high-end systems (and priced accordingly). Its like saying a Cray sucks because it was built 5 years ago.