Sirius Travel Link won't be backwards-compatible with previous systems. It requires new hardware. The new Ford navigation system, which features Travel Link, is hard drive-based. Current systems are DVD-based. The 2008 MKX with nav and Sync will give you good options for hooking up your cell phone and MP3 player, but it won't be able to show traffic, weather, and gas prices from the Sirius Travel Link service.
I've been doing some research and it seems that the new Sirius Travel Link service has been getting rave reviews. Unfortunately it is only available on a select few 2009 models: Ford Flex, Mercury Mariner and Lincoln MKS. I'm trying to figure out if Sirius Travel Link may be made compatible with any additional, older models sometime in the future? Specifically I'm looking at a 2008 Lincoln MKX that comes with navigation and Microsoft Sync.