If you are using a 2 GB microSD card, the simplest way to try the latest release is to download a bootable image that you can copy to your microSD using your development machine's dd utility.
These prebuilt images are available at:
http://www.gumstix.net/overo-gm-images/
Download the image (beta-2.img.bz2 for this example) and then uncompress it:
$ bunzip2 beta-2.img.bz2
You will now have a file named beta-2.img which should be exactly 1900000000 bytes in length (it is somewhat undersize in order to fit on 2 GB microSD cards with bad blocks)
Now insert your microSD card into the slot on your development machine. This example assumes that the card mounts at /dev/sde. If it mounts at a different spot on your machine, you will need to adjust the following commands.
$ sudo dd if=beta-2.img of=/dev/sde
Now unmount the microSD card.
$ umount /dev/sde
You now have a microSD card with 2 partitions: a bootable FAT partition with MLO, u-boot.bin and uImage; and an ext3 partition containing the root file system.
Insert it in your Overo's microSD slot and apply power.