I just found this really good deal in eBay. The memory upgrade that I needed for my P1610. What's good about this deal, well, the price. So far the memory for the P1610/20 can only be bought from Fujitsu at a price of more than $400. This one is for $129 including shipping. According to some comments I found in a forum, it's confirmed, it works in the P1610/20.
I just placed my order.