In Pages, try save as PDF from your printer dialog box. Choose PDF-X, which is a common PDF profile used for commercial printing. See if that makes a higher-resolution document from which you could export out of Preview.
I don't have Pages on my computer, but I tried doing a save-as out of Preview. If I start with a 300ppi file, the end result is a 300ppi file, according to Photoshop. So, if the PDF-X thing works, you're in like flynn.
Other than that, I agree, Photoshop is the uber-app for that sort of thing. Good luck!
Jeez, you'd think they'd give Pages the capability to output a high-res file, since they bill at as their answer to Publisher. If you're going to do a lot of commercial printing output, I suggest you invest in the Adobe Creative Suite of applications, or at the very least InDesign and Photoshop. They are full-featured and will make your printers' lives much easier. (Or, you could hire a professional graphic designer, like me. <g>)
It just occurred to me--most commercial printers can take PDF files. See if they can take the PDF file that you generate from Pages using the PDF-X profile.
Good luck!
--Dina