IBM has great laptops. The T43 should give you something a little faster than your 600m (depending on old vs new specs) and still be about the same size/weight. I also have a 600m (1.8GHz) so I'm wonder what speed yours is that you looking for a new machine. If you already have a decent speed CPU I'd recommend getting a little extra ram and weight until you can afford a dual core mobile solution. I think IBM T series may have a dual core now but Dell only has the tank size laptop now. I picked up a refurb XPS M140 for not much but will likely end up selling it and keeping my 600m. Going to 1280x800 from 1400x1050 is not easy.
I vote IBM ;)