First I found a way to connect wifi in p900.
The hardware is in this links http://www.pdalive.com/showarticle.php?threadid=4598 & http://www.mobileplanet.com/product.asp?cat%5Fid=102&cat%5Fname=Palm+OS+Handhelds&dept%5Fid=5040&pf%5Fid=MP730000&listing=1
BUT its a memory stick connection...
Hmm after a little search I found a solution, here http://www.clubsonyericsson.com/en/products_uaccesories_armmsjacket.shtml
But that solves only the hardware part, I dont know zip about any drivers or software or battery consumption. also, have in mind that you will not have hard drive to save any data...
So please, if you find any info please tell us!!!
About the second part, here in Greece we have a store that upgrades p900 to p910 sofware with 25 euros (good price) that enable us to use MS larger than 128MB Yeah!!!! But , we have a problem also, due to to the different chipsets between p900 and p910, we cant use bluetooth headsets, but connection with PC and other devices Excluding voice services work normally!!!