I'm the guy that did that m5550 video card swap.
I ended up putting the GO7600 back in, and returning the ATI x1900
The ATI was significantly faster, and really made gaming (at decent settings) possible on the m5550.
However, the heat it put out was FAR too much. I was forced to use a laptop cooler ALL the time, and even then the case became brutally hot.
Now, if someone would relase 8600 M's w/ 512 MB that would boot in alienwares, then I would try it again.
But until then, you are pretty much stuck with the crappy GO7600 if you own an m5550