[graphicgr8s]

If the lab will scan them at hi-res then just let them do it. If not then get them printed as large as you can afford ie: instead of getting 4 x 6 get 5 x 7 or even 8 x 10. This gives you a better starting point. Get yourself an Epson scanner, runs about a grand. Scan at the finished size you want at 300 dpi. this will print just fine. My customers usually only bring me a 4 x 6 and with correct scanning and resolution I can get up to 16" x 20" with great quality and very little photoshopping. It's what I do every day for a living.
Use to have a darkroom back in the late seventies. Had it for about 7 years. Did 4 x 8 foot murals and had to mop on the chemicals (color) it was a blast. Wanted to get my old stuff out again and set up a DR but the stuff is too much$$$ and the EPA is always on your case with the chems.