You will see 100% of the card's power with a pcie 16x 1.0 slot. The reason is that 2.0 just increases the bandwidth to x2 of 1.0. So, a 8x pcie 2.0 slot is just as fast as a 16x pcie 1.0 slot. At this point, I don't know of any single gpu card that actually uses all the available bandwidth of a 16x pcie 1.0 slot, so giving it more bandwidth (going to 2.0) won't matter.
As to HDTV processing, I know the newer cards handle MPEG2, H264, and WMV acceleration. Not sure what else. I don't know if the 2600pro uses hardware acceleration for all of those video types, though.
And yes, most all cards out right now are pcie 2.0, since last year or so. The extra bandwidth will most likely be used in future cards.