Poster Printing???

[Dmc]

I am often hired to make large printed posters from 8 1/2 x 11 sheets of paper. I do it 2 different ways. A low-resolution and a high-resoluten. My low-res way is quite simple because my printer has a built in poster preset. It works fine for simple images. but for detailed large images its just too blury and pixelated and I find my current system for hi-res very painstaking and long.
1. Convert the image to the exact size needed in photoshop
2. export the image as a BMP
3. Open BMP in Paintbrush
4. Print the image to an image driver (like Zan Image Printing)
*This step is neccessary because my printer cant handle, for some reason, 1.81 GB of data at a time*
5. after 6-7 hours I print each of the 42 (for a 6x7 page poster(i think)) images one by one and put them together.

Does anybody know of any software (comercial or freeware) that will speed this process up???