The 200-series nVidia cards do not have HDMI outputs, only HDMI 1.1 via a pass-through adapter attached to the DVI ports. The current revision of HDMI is 1.3. I'm not exactly sure about the differences, but I'm pretty sure bandwidth is limited for 1.1 and it only supports a refresh rate of 60Hz. I used such a setup to a DLP television on my 8800GTX and the display was not good: the screen was far off center and couldn't be adjusted, and there was visual noise on the screen.
The WDM model in Vista and above does make display connectivity a lot easier and better. XP is also ten years old now, so an upgrade is in order regardless, not to mention the fact it only supports DirectX 9.0c and is no longer a priority for Microsoft to update. I would go with Vista instead of 7, but that's just me...
The GTX 570 has a wider memory bus providing more bandwidth, which is important if you want to run games with higher levels of antialiasing. Apples-Apples, the GTX 570 is probably 15-20% faster. Real world gaming however is a lot closer, maybe 5-10%. Now that I think about it, the fact that you can find a GTX 560 with 2GB of memory makes it more appealing...
[H]ardOCP Reviews:
GeForce GTX 560 Ti GC
GeForce GTX 570