I've never heard of a monitor running off usb - and I don't believe the interface is even fast enough. If you have any links to monitors that run off usb I'd be fascinated to see them. Have you ever seen a graphics card with a USB output?
When monitor specs mention USB they usually just mean a usb hub or pass through to extend the range.
No USB out on your graphics card, so forget USB. Yes, HDMI to DVI cables are very common. I haven't shopped around, this is just the first one I came across as an example:
http://www.scan.co.uk/Products/2m-Sc...28CDLDV-302%29
They essentially just ignore the HDMI digital audio signal and pass the digital video signal through which is compatible.