tell you what I do , since most printers line screen out to 150 lpi anyway, when I make up billboards I do the artwork at 10 % of the final size as a uncompressed tiff file at at least 300 dpi , 450 dpi if you have the machine to pull it off, take the final render at that size convert it in photoshop to cmyk , then in in design , freehand or illustrator take it up to 50 % of the final size once you insert the tiff, then just tell the printer to play it out at 100 %, gaurenteed it will look awesome , most large format billboards get printed out at 80 dpi at 100 % anyway, if you do the math you will see 450 dpi at 10 % works out to pretty much the same, besides your one saving grace of large format prints is 99% of the time nobody is close enough to see the pixliation , and the distance they look at it there eyes blend out all the artifacts anyway
If you are using max try finalrender it can handle big render quite well it caches to hard disk so it can handle huge tex maps and huge render images on a slow machine without taxing the ram