Give the demo version of Qimage a try. It contains some of the best
upsamplimg algorithms around, is a fully managed color application, and in
general a swiss army knife for printing.
The latest version of Genuine Fractals (version 5.0) is amazing. I was
just on a speaking tour with Adobe and OnOne (I am a consultant for
hp). I got to work with Genuine Fractals first hand and make 1000%
upsamples which were printed on a Designjet Z3100. The results were
stunning. I recently ordered my copy and it's due to arrive shortly.