For other more 2D video centric apps X1600 > X800.
PS3.0/SM3.0 features will be as unuseable at normal resolutions as it is on the GF6800 series. The implementation may be more effecient, but it's not enough to overcome the fact that it's still a mid-range card. Of course it will allow you to see the slideshow of SM3.0, but not use it in games effectively.
So for games, the X800XT at the same price would be better. I still say wait the X1700 on the PCIe side of things, but who knows when that'll reach the AGP market if ever.
As mentioned, the X800XT EASILY out-classes the X1600pro when it comes to power. I might say on a power level, the X800XT is nearly double the X1600pro. (the X1600pro is a bit above the X700pro) As for SM 3.0, anything that efficiently manages to implement HDR is programmed carefuly enough that it can be done with SM 2.0 or 2.0 extended anyway, so SM 3.0 is moot, as GGA said.
And, of course, as mentioned, there's AVIVO to consider; it looks highly attractive for the 2D video buffs among us. And believe me, normally, video encoding can take a LONG time through software. Which makes it the kicker that my brother, who's doing a lot of video editing, has an AGP X850XT PE, and replacing the current CPU and motherboard would be out of the question. Now, if only ATi made some "AVIVO-only" boards for plain PCI... (perhaps included them in their "TV Wonder" cards)