If an image file is compressed, it's no longer a raw file. If some kind of processing has been done, you don't have a raw file any more.
You could help things a bit by using 4 GIG memory cards. Once the image files are in your computer you could do whatever conversion you like by using a script, applet, or droplet.
I don't know how many pictures you take when doing a wedding, but, would it be possible to have enough cards to leave all of your images on cards until you can put them on your computer at home.
If you do much wedding work you may want to get some more storage capacity for your computer. As a matter of fact, I would want to have a RAID array with shadowing of drives. Being a cautious sort of a fellow, I would also want a backup system running all of the time. Just imagine how you would feel if that 80 GIG HD died. Imagine calling your customers to let them know that you won't have any pictures of their wedding. And for your last bit of imagining, picture yourself writing checks to refund the deposits you had collected from your customers.