Here is a little tutorial for you. You will need to download Wiikey DVDDump v0.8 or higher and either a gekko card or a home made gamecube>SD card reader.

(I am just going to paste the readme from hereon because it does a fabulous job instructing us how to use the program!) Please do note that this is the readme for version 0.8, higher versions will probably have more features. Just something to look at for now

Most cards other than sdhc should work. We tested various cards from 512mb
to 2gb. There may be some non-sdhc 4gb cards, but we didnt have one to test
with. Cards smaller than 512mb should work, but are not recommended as they
will require too much swapping.

Running dumptool.exe requires administrator privilege. Please login as an
administrator or member of the administrator group.

Before using your sdcard for the first time, you need to format it using
our tool. To do this simply run:

dumptool [driveletter] -c

This will format the card to fat16 and allocate space for the dumpfile.
It will use all but 10Mb of the available space, in case you want to put
some other applictions on there as well.

Depending on the size of your card the dumper will split up the image to
multiple chunks. Once a single chunk has been dumped, you must download it
to your computer using dumptool.exe.

There are two methods for downloading the chunks to your pc. Using the
option -sd will store the data into a single image file called discimage.iso.
This is the recommended way, because you dont have to join the parts after

dumptool [driveletter] -sd

will get the current chunk from the card and append it to the image file

Alternatively using the option -d will create a single file for each chunk
ie. 00.rvl 01.rvl etc. You have join the parts manually when using this method.

DO *NOT* use multiple cards when dumping a single disc, it will result in bad dumps!

A complete image of a gamecube disc should be exactly 1459978240 bytes
in size. The correct size for a wii disc image is 4699979776 bytes.

Using the batch files:
If commandlines scare you, you can also use the provided batch files
formatSD.bat and dump.bat.