The performance difference between laptops and desktops is not really large today... with the exception of graphics cards. I was thinking of replacing my gaming desktop with a laptop, and using some external graphics card adapter to enable it to use a more powerful graphics card - for instance, one of Village Instruments' ViDock series, which connects using an ExpressCard interface.
However, I'm wondering - is there a performance impact of connecting a graphics card like that, via an ExpressCard, compared with the performance of connecting them via PCIe in a desktop computer? Is the bandwidth comparable? Is the latency comparable? etc.