Pretty simple solution. I have a 24" monitor, wireless mouse and keyboard sitting on my desk. Either my wife or I can plug in and be running using the extended desktop feature with both the big screen and the laptop.
It's easier (and more comfortable) to sit a laptop on my desk when needed than to sit a desktop PC in my lap on the sofa.
Some years ago when the company IT department finally agreed that I needed a new computer, rather than supply a laptop (necessary)
and a desktop workstation, they went with a high-end, tricked out 17" Dell Precision laptop, a docking station with more room for cards and another hard-drive, 24" monitor, keyboard and wireless mouse.
The laptop was as powerful as many high-end workstations, and worked well with CAD software with both the monitor and laptop screen used together.
Of course, this was a fairly expensive system (~$6k without software.), and it was a very heavy laptop to take on the road, with limited battery life, but it did the job.
Currently I favor the i5 and i7 Intel processors, Gigabyte or Asus motherboards, and Windows 7. My current personal laptop processor is a bit older, but it still lets me run a virtual machine with Windows XP (Free from Microsoft) to support some legacy business/school/engineering software that weren't able to run on the main Win 7 installation.
Another big consideration is whether or not there are drivers available for any older peripherals you are still using. I had to work pretty hard to get my flatbed scanner working again with the upgrade from XP to Win 7, and while I managed to get my TV input card working finally, a Windows update killed it again and I eventually had to get a replacement card that had the Win 7 drivers.