When I last looked at low-end projectors a few years back, the cheaper options suffered when it came to brightness, contrast, native resolution, quality of scaling, and noise.
Despite having native resolutions of 800 x 600 and 1024 x 768, respectively, it looks like both projectors can handle scaling HD signals. This may help explain and illustrate what your screen will look like as a result of scaling, though doesn't address the quality directly (only the # of pixels in play as an example): http://www.theprojectorpros.com/learn-s-learn-p-theater_aspect_ratio.htm
FWIW, I've seen an old 640 x 480 projector displaying HD and I thought it looked terrible. I imagine 800 x 600 also wouldn't look very good. I've seen 720p scaled to 1024 x 768 and it was fine. Not outstanding, but fine.