Hi Smallboy!
Congrats on your new monitor!
moving to a 6600 won't make any difference image quality wise, since you, as you pointed out, play no games at all. The DVI signal transmits losslessly and the internal graphics chip on the x300 calculates colors and its corresponding bits and bytes entirely digtal anyway, so it's just as good as any other card with DVI
.
The only reasons I could possibly see for you to upgrade your graphics card would be TiVo functionality, in case you missed this in the X300.
Or maybe if you found your card to be too slow for scrolling down large documents (PDFs or HTML/Flash) pages without things starting to get jerky. But I'd be surprised if a X300 turned out to be too slow for these tasks.
Hmm dunno, what else ?
Maybe HDTV hardware decoding?