Enlarging pictures

I've got some renders from Cinema 4D that I want to use for a banner at school. The problem is that the renders are all 1024x768, and the banner is something like 3 ft by 6 ft. How can I make these pics big enough to use on the banner without the losses in quality that come with stretching an image? I've tried simply blurring them, but there are some layer styles used and those don't blur. What should I do?

Short answer: You can't.

Re-render at about 10, 800px for the 6' dimension and you'll have enough data to render a great image at that size.

yeah for printing you don't wanna drop below about 275 pixels/inc for a banner that size so 275 *12*6=19800 by 9900 (half of 19800)
FINAL IMAG SHOULD BE 9900 X 19800PIXELS AND set it as 275 pixels an inch... or you can do 300

its all good now, I did some tweaking and rendered the images at the actual size of the banner so now everything should be all gravy and if not I'll just blame kinko's