I don't work for IBM but spend a lot of time with their product line.

Right now the T42 shows as being a valid model till at least mid Q4. That is as far out as I can see at the moment. The next rev of the P-M chips from IBM might change things a bit but for now that is where they are. If you look at the Thinkpad lineups for the last few years they usually get to the third model number before the refresh.

RAM is a hard thing to look at as an investment. Prices change often and having more is always a good thing. My theory is to buy what you need/want now and if it works in the next rev machines you are that much ahead of the game.