Alright, I work for a Print company, so here's some advice:
1) If it's going to be screen printing, the highest lines per inch will be 150 -- so essentially, you'll need only 110dpi to 300dpi, depending on the final size of the image. If it's digital, it depends on the output of the printer, but building at 300dpi would be safe.
2) You should not size 'up' your graphics. This will, as people have said, cause blurring. If you're graphics are at 72 dpi, and you size them up for printing, they will come out like hell. You need original files, and build them at 300dpi. They won't go larger, and then you can size them down later if you need. It may seem like it's ok with filters -- but if you test print out something, any error you have will be magnified.
3) If you are going to 'swing' you're color over to CMYK, contact the printer you will use and ask them their specific color settings. If you cannot accomplish this in GIMP, then give them the raw RGB files, and let them swing it. Us print designers have specific 'color gamuts' that transfer our RGB files to CMYK for printing, and they vary depending on the type of printing we are using. If you swing your color to your CMYK, you may lose quality if your gamut (or range of color) is much smaller than the final printing process.
4) Most print shops do not like just rasterized files (such as jpegs). We prefer Tiffs, Photoshop files, rasterized EPS files. Any text (that does not have design elements like photoshop techniques) should be done in a vector art program (such as illustrator, freehand, corel, etc.). To combine the two, you can do that in the vector program, or for professional version, use Quark Xpress or InDesign. The vector text is important -- especially when using digital. The quality is largely improved.
So, I would recommend finding the size of the print (inch x inch) and start rebuilding your layout at 300dpi (or less if it's screen print) so that it prints correctly. Build your files at RGB -- but remember that some colors will not print accurately CMYK. RGB sees 13% of visual light and CMYK sees 7% of visual light. Colors, especially bright blues and reds, cannot be accomplished through just CMYK alone. Colors that are PMS (Pantone colors) can though.
You can view some info about color at http://www.pantone.com
Otherwise, I don't have any resources to give you. Much of this is vastly different than the web, and print designers know little of the web for that matter, so not much that is highly professional online. Should you want to talk about this further, send me a PM, and you can call me here at work. We can talk about how you should build your files more in detail.