Hello. You will want to make your files at least 600 dpi for them to look their best when printed commercially. Save your original graphics files in a lossless format, like TIFF or PSD. The printer you choose will probably want to print from an EPS file, but you can save your TIFF or PSD as an EPS when you are finished with it.
Standard card size is 2.5"x3.5", and standard Bridge cards are 2.25"x3.5". If you are making your cards 2.5" x 3.5", increase the size on all sides by 1/8" to account for bleed (so your actual file size will be 2.75"x3.75").
Make your cards using CMYK color. If you use RGB, they will look very different when they are printed on a commercial press. Use a "punched" black (C40, Y30, M20, K100) to get a deep, rich black. Find a Pantone-to-CMYK conversion guide to check that your colors will look like you want them to. Colors that print fine using a laser or bubble-jet printer may look very different when run on a commercial press.
Do not set your type in Photoshop. Use a program like QuarkXpress, PageMaker, or InDesign to add text to the cards. Create the document with one card per page (page size set to 2.5"x3.5"), import the EPS that is your card background, and add type over that. This will ensure that the type is crisp when printed.
I looked into more than 20 card printers for pricing estimates before I went ahead with my product, and the two that stood out were
Chicago Game and Card and
Delano Service. The prices were comparable, and I went with Delano Service because it is within driving distance of my home. My project manager at Delano was very helpful, and I can send you her e-mail in a PM if you would like.