A hint for you.
You can "clone" your screen to get two identical images on HDTV and monitor, but unless the tv has exactly the same resolution and size it is unlikely you will get video to both screens at the same time. That notwithstanding it will work fine otherwise.
Dont be fooled into thinking that the HDTV will have a better picture than the pc either. This is definately not true in many cases. Monitors by their very nature are and have been for quite a while HD. A larger screen can actually make a picture worse unless you are running very high resolutions. You may then find that the TV cant show these very high resolutions. Mine will only accept 800xs600 but my monitor is at 1650x1050 - see the problem! If you clone a screen they need to be the same size which can affect quality on the large screen or make your monitor resolution too low for a good desktop.
As for the card, well there are many cards that have dual dvi and at that budget there is probably not much in it.
Go ahead with the card, but just extend your desktop on to the new screen effectively giving you one huge monitor. Then just move your slideshow across to the HDTV.