This might be old news to some, but it's too valuable to not pass along. Tony 'Nicoya' Mantler's SGI Video Mode Page provides instructions for generating two additional video modes on many SGI machines (Indigo2 IMPACT, Octane, Fuel, Onyx IR/VPro and O2): 1408x1024 and 1152x870. (Indy, Indigo and non-IMPACT Indigo2 graphics cannot take advantage of custom modes.)
Although Indigo2 IMPACT can display 1600x1200, I find it's not really usable on a CRT due to the 60Hz refresh limit. The standard 1280x1024 mode has an adequate refresh rate @ 76Hz but it's a bit narrow ... not to mention the need to crop standard 4:3 aspect wallpapers to fit. Nicoya's 1408x1024_72 and 1408x1024_76 modes are quite nice; providing more desktop workspace, a high refresh rate, square pixels and compatibility with all 1280x1024 capable monitors. The thumbnail at the start of this article links to a screenshot of the 1408x1024 mode with a 4:3 wall from Gamespot Japan as the background.