By the time the N900 is out, SDHC cards will be 16GB or more in size - why wish for power sucking HDDs when you can store all your data in battery friendly flash? Also consider the Bluetooth/WiFi equipped HDD companion devices such as the Bluonyx that will allow you to carry your data in centralised storage. I would say it is a given the N900 may use a more powerful CPU, perhaps an OMAP3 but I'm not sure it's necessary right now given the level of under utilised hardware already present in the N800 OMAP2 (IVA, PowerVR MBX). We can only hope Nokia see the light on USB connectivity.

If/when the N800 acquires better media playback, it may become your ideal one-stop device (with a couple of jumbo SDHC cards!)