I'm the same as a few have stated already. I shoot with two cameras and I've got 1x 8GB and 5x 4GB Sandiscs. I keep the 8GB in the 1Ds, thanks to those big file sizes it munches through it at a fair pace. I also stick a little SD in it and write JPEGs to that. It provides that instant backup and although the files would never make A1, I won't be missing them altogether. Just a little bit of piece of mind since I've never had a card fail.
Once the 8GB is getting somewhere near, I'll swap to the 4's.
The other trick I have is to actually use both cameras. If I'm shooting a wedding, I'll shoot bridal prep using both, that way if anything happens to either card or camera I still have a quantity of pics on the other. One on each shoulder and spread the load. I do the same with two different focal lengths during the ceremony and a second shooter is even more insurance!
The 5D gets the 4GBs and gets used throughout the day. I find that because it is lighter, I actually prefer using the 5D if I'm doing any one handed off camera flash such as first dance.
As for managing the cards, I have two in the cameras and I have a waterproof case for the spares. I keep them turned face up if they are formatted and ready to use and face down if they are used. Easy peasy card management.