I really don't think it's worth it to upgrade the laptop GPUs. From what I have seen, the laptop GPUs are very very expensive, maybe $400 or 500+ for a relatively dated laptop GPU.
Probably better to sell the laptop and buy a new one.
Anything that's cheap and got a good cooler, most companies won't BS you on RMA/warranty.
I got a Galaxy 480 and it went bad a few months later, RMAed it and received my new card within a few days.
I know some of the GTX 570s are around $300 so if you can find one I would go with the 570.
I have a MSI reference 570 right now and it's 1.2GB is enough for most if not all of my games at 1920x1200 high setting.,
I have 6850 CF with i7 920, and I get well over 60fps in BFBC2. 1920x1200. All high
However I do recommend GTX460 1GB SLI over 6850, only because 6850's drivers sucks.