GT4 - limited edition, came in yellow (as well as black, which was a standard colour), plaque and if you were lucky a games console in the back, "GT4" alloys (heavier than Rays, not forged and the standard rims for the rest of the world)
MY06 Facelift - revised (and many argue much improved) interior, revised front bumper. led rear lights, Rays as standard
Both got the rev up 296 bhp engine.
I have to agree with the others, I can see no reason why a black GT4 would be more desirable in the future or indeed why anyone would go out of their way to get one now. Admittedly, they were very desirable 5 years ago before the MY06 made the standard car better again. If you had a particular penchant for yellow then at least there is something unique about a UY GT4 but otherwise what's the point?