400MB should be enough for anyone! But just in case it's not enough for you, this is how you can resize your SDE image.
Connect the Archos device to your Linux computer via the Archos-proprietary USB cable. Your computer should automount it for you.
Open a terminal and perform the following steps, replacing <NEW_SIZE> with how many more megabytes you want the new partition to be:
dd if=/dev/zero of=/tmp/new_image bs=1M count=<NEW_SIZE>
You have created an empty padding file that you now add to the existing image.
Now if you have an Archos 5, run:
cat /tmp/new_image >> /media/ARCHOS5/rootfs.img resize2fs /media/ARCHOS5/rootfs.img rm /tmp/new_image
Or if you have an Archos 7, run:
cat /tmp/new_image >> /media/ARCHOS7/rootfs.img resize2fs /media/ARCHOS7/rootfs.img rm /tmp/new_image
Please note: If your Archos is using VFAT (aka FAT32) as its filesystem then the maximum total size of rootfs.img is 4GiB. This is a limitation of this filesystem type.
Also keep in mind that the size of the filesystem increases but the number of available inodes doesn't. If you install packages containing many files you will still run out of inodes. This is to be fixed in the near future.