Toyota recommends the following Iridium spark plugs...
Iridium plug, 14mm, 5/8" hex, .044"gap 1ZZ-FE (2000+ Celica GT)
DENSO - SK16R11
NGK - IFR5-T112ZZ-FE (2000+ Celica GTS)
DENSO - SK20R11
NGK - IFR6-T11
Iridium is a precious metal that is 6 times harder and 8 times stronger than platinum, it has a 1,200=F higher melting point than platinum and conducts electricity better. This makes it possible to create the finest wire center electrode ever. Prior till now, spark plug manufacturers have favored platinum for their long life or Performance spark plugs due to its high melting point, also the technology did not exist to machine and bond iridium on a spark plug electrode(at least in a cost effective manner). Champion spark plugs had already made an iridium industrial application spark plug, but it still sells for over a hundred dollars per plug. Just now is the technology available to effectively use iridium in a spark plug for automotive applications. The strength, hardness and high melting point of iridium make it very well suited for a fine wire plug. The primary iridium plug manufacturers at this time are Denso with a 0.4mm center electrode and NGK with a 0.7mm center electrode. Both are the best Performance plugs on the market for traditional automotive use and many racing applications.
SIDENOTE: Thus far the tech's we have spoken with report no problems using iridium plugs with Nitrous.
Iridium Technical PDF...http://www.sparkplugs.com/pdfs/iri.pdf