With the El Gato brand USB TV tuners, the only major difference with most of them (other than obvious hardware features) is the software that comes packaged with them. A couple years back I was able to get a friends TV tuner working after he switched to a Mac by installing the Mac OS software that otherwise didn't come with his product. It seemed unfair at the time that he would have had to spend another ~$150 to purchase a whole new product. He did originally purchase the tuner not too long before, after all.
I'm not sure if ATI even makes a Mac compatible version of their TV Tuner though so that trick might be moot.
If it's going to work in Parallels, you'd plug it in, make sure Parallels is controlling the device (make sure it is checked in the Device/USB menu) and install the software for it, but I suppose that comes down to Parallels DX support, according to old-wiz which I am otherwise not familiar with.