a good USB3 flash drive (The USB3 stick is really good. The random reading speed is 98MB/s.) formatted as Ext2 (Ext3 is not good for a flash drive since it constantly writes its journal to the same sector dramatically reducing the flash drive life time) and made the GRUB boot from it. GRUB sees the stick as (hd2). No extra steps was required.
The "/var" directory I left on the HDD, mounted to a partition which was prior that a swap space. I think I don't need swap with 8GB memory, right?