I don't know enough about laptops to recommend any, but I'll answer what I can.
Firstly, Core Duo 2 is faster than a Turion X2, and has sligtly better battery life. Turions however are cheaper, and in the real world if you aren't using intense software like brand new games or doing CAD work then the difference is minimal. With your budget though you probably could get a Core Duo 2 anyway.
While you can upgrade just about any component in a laptop, it is usually hideously expensive to do so to any components other than the mentioned two.
And as for the DDR3 thing, don't worry about that. There is absolutely no real advantage of DDR3 over DDR2. Or DDR2 over DDR for that matter. Either way, that is irrelevant.
And yes, Toshiba is a good make