Need a Printing expert (DPI&PPI)

Hi guys first post here. I'm having trouble with an extremely large document and I'm wondering if it can be any smaller.

I need to design a large logo for a billboard ad. The dimensions are about 14'x6' I spoke with the printing company and they weren't all that helpful about DPI & such. The most info I could get was that they print at 150 DPI, and if I wanted to make my image smaller they will scale it up.

Problem is, it's a very elaborate pixel based graphic that I don't want to be distorted when blown up. So, I've been trying to design at the literal dimensions of the sign, and 150 pixels/inch (in CS3). Problem is that I'm getting all kinds of scratch disk errors (even with 250gb free) and extremely long loading times between tasks.

Has anyone printed or designed for something to be printed at a size this large? I've made business cards before and those are always made to spec, 3"x2". I'm not 100% sure I have to follow this same rule. If it were a vector design it would be a lot easier but it unfortunately has to be pixel based. Thanks a million if you can help.

Make your graphic at a proportion smaller than the 14' x 6' and make your ppi higher.
Example: If your final size is 14' x 6', make your actual graphic 7' x 3' at 300 ppi. The end user should be able to resize by unchecking "Resample image" in the image size screen and inputting 14' in the box instead of 7', and the ppi should automatically change to 150.
What you are actually doing is creating at a smaller percentage, to be enlarged. This should help with your disk space issues, also.
I've created large posters in this way, without any loss of detail. 150 dpi from a distance will look great.
Any questions, please leave me a message.