Peter we have that inspiron in-house with the Radeon chip and it's a very capable system.
The primary issue for laptop graphics is not the graphics chip so much any more (since all new chips are quite powerful compared to a few years ago), but instead the issue is the screen refresh rate. Most laptop screens run at 60hz, which creates a horribl environment for IRONCAD. Our inspiron runs at 120hz and so the refresh rate when orbiting a scene in IRONCAD, for example, is very nice. So whatever you do, insure that you can find a laptop which has a refresh rate that is higher than at least 72 hz. (IE, the best graphics card in the world will perform quite poorly on a 60hz screen)
I can't comment on how the laptop will perform through the external monitor. You may not be able to get the higher refresh rate through the external port; I don't know. I don't know if you will experience enough lag through the external bus that the more powerful Quadro2go chip would be justified. If I can find a minute of free time today, I'll try to test that locally.
So we're very happy with our Insipiron with the Radeon chip. While a real benchmark would show you that the radeon would outperform the geforce by a minor amount, there really is no noticeable difference between those to graphics chips.
As for using a laptop as a primary workstation; that's a completely different issue. Laptops have far slower hard drives (so files take longer to load/save), a slower system bus, and they hold less ram. Definitly put as much ram as you can afford into a laptop in order to compensate for the other slow components.
By the way; our inspiron has the Pentium 4 chip in it. We have not yet used a Pentium M chip, but I have heard many reports that while the Pentium M clockspeed is lower, it still performs just as fast (an in some cases faster) as the latest Pentium 4; and it's better for a laptop because of heat and battery requirements.