If you aren't planning on using video chat on MSN then I would just recommend sticking with telepathy-haze and butterfly. Telepathy will integrate with the existing UI.

I believe you can use the command "df" (without quotations) in x-term (the Linux terminal in the device) to see how much space is free and used in each partition.

Sadly, I don't have the n900 myself (sadly may not get it till next year with the way Amazon is acting).