The N900 and Fremantle (Maemo 5) are synonymous with each other - there is no better device for Fremantle and no better operating system for the N900. Although there are some irregularities with the hardware across all N900s, they're enough the same to be considered as much - there are no versions of the device. The latest OS releases are (as mentioned above) obtained from the CSSU repositories.

You can tweak/edit/hack so much with this pair (device + OS) that almost anything's possible, within the limits of its hardware.

What sort of functionality were you hoping for on a handheld? Perhaps someone here can guide you to achieving it?