I was looking for this for a while... I need to input power point presentations that clients want to operate themselves, without option to convert the content because they usually have it ready 1 hour before the show...

I tried the canopus twinpact100 but the delay on the firewire is a problem....
the best I've used are the epiphan USB2VGA units they have a wide range supporting different resolutions..
I use two of the LR version, 30fps, at 1280x1024 top resolution.

check www.epiphan.com
The will run you about U$800