mandrake,

Those are the latest firmwares so yes you don't need any previous firmwares. Download those 2 images and follow the instructions on the wiki page to flash them.

You will simply be updated to the latest version when you upgrade firmware. Search around for PR1.3 changelog.

CSSU-Thumb is explained in the thread I linked too; its not an over-clock but a re-compilation of apps and libraries using the Thumn2 instruction set which makes them smaller in size compared to how they were previously compiled hence improving performance of an ageing device quite a bit

Good weekend to you too