Apart from the Pro running at 500MHz GPU/800MHz RAM, and the XT at 578MHz GPU/1.38GHz RAM, they are the same card.  The XT does tend to be more expensive, and faster, but HotHardware wants to know which is the better bang for your buck.  Handy hint, they are available in both PCIe and AGP.
“Selecting the right video card when you’re on a budget can be difficult. The major deciding factor for many gamers out there is price. As mouthwatering as it is to think about getting a top-notch SLI or CrossFire setup, it’s a luxury that most simply cannot afford. Fear not though, as both Nvidia and ATI have some quality entry-level and mid-range graphics cards in their respective line-ups. Today we have two such cards with us, Sapphire’s Ultimate X1600 Pro and X1600 XT. Aimed towards consumers looking for a silent video card at a budget price, Sapphire’s Ultimate X1600s will appeal to those looking to build up an HTPC or a PC for casual gaming.

Both cards are based on ATI’s RV530 GPU. The X1600 Pro and X1600 XT have 12 parallel pixel pipelines, 5 vertex shaders, 4 texture units, and 4 render backends (ROPs). Both cards also support DirectX 9.0c, OpenGL 2.0, are able to render cutting edge features such as HDR lighting with AA, and they are both CrossFire ready. The only real differences between the Pro and XT are clock speeds. ”

Here are some more Graphics Card articles from around the web:

Click Here to go to Video Cards   Graphics Cards