I second the advice of Klanger.
Your first post said any /BSD. A usb install image that will fit your 256mb usb drive is available for netbsd, openbsd and dragonfly. The FreeBSD memstick .img is around 900mb.
Of the three I would recommend OpenBSD. In my opinion it is the easiest of all to install. The FAQ gives you step by step instructions and it is up to date.
http://www.openbsd.org/faq/index.html
The hardest part is partitioning and disklabeling the drive and defaults are available to simplify the task. Do you have broadband?