400MB should be enough for anyone! But just in case it's not enough for you, this is how you can resize your SDE partition.

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>

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/ARCHOS5/rootfs.img
resize2fs /media/ARCHOS5/rootfs.img
rm /tmp/new_image