According to my old magazines, that bottom Buick, called the Velite, would have had a 400hp twin-turbo V6. I don't think it had anything to do with the Sky/Solstice, as it's a lot bigger and like I said, their two concepts didn't show up until a year later.
But the one I really like is that coupe/convertible thing right above it (in the picture with two cars). It was going to be called the Wildcat Gran Sport and have a 425hp V8. Supposed to go on sale in Fall 2008. Too bad about that.