Unitl it's full of outstanding RAW images, and you have 4 more hours to shoot!
I have 7 cards at 4gb each, and 2 more at 2GB. Also, don't get the cheapies. Look for the 133x or better. The utlra IIIx and IVx SanDisk are my fav's, but there are many other quality high-speed cards out there. The advantage of the high speed cards is they allow your camera buffer to empty faster, and keep you shooting. If you're shooting action or doing a lot of bracketing, this is important.