Photoshop :: Sizing Up A Jpeg That Has Artifacts For...

I have some really good resoloution images for a large 8x4ft poster project. but one of my clients larger images (an important one) is just a 1000 x 1000px image @ 72dpi that has some artifacts which needs to occupy a approximate 25 inch square on the final print. I can't get a better quality image than this from the client at this time as they don't have it. what techniques can I use in photoshop to increase the print size while improving the final printed image. In the past, I sized up the image to the size needed and set the dpi from 72 to 300dpi (using "bicubic smoother"), add some surface blur, add monochromatic, uniform noise @ 2% +/- (to help smooth out the artifacts). with this image now, there are a lot of flat shiny surfaces where the artifacts are more noticable, etc. what kind of techniques would you use for this kind of job? (apart form hitting the client xD)