Using "sips" to change resolution of PDF file

Within the MacOSX Tiger terminal, I can use sips (scriptable image processing system) to convert a PDF to a JPEG. However some of the PDFs are 150 dpi and I need the final JPEG to be at least 300 dpi. How can I use sips to change the resolution of the PDF to 300 dpi before I convert it to a JPEG.

Thanks in adance for your help.....Cassj