Believe it or not, I'm actually using a cheap e-machines laptop. Don't need a high priced laptop to view radar, be on spotter network, and possibly stream video. My laptop gets the job done and that's all I need.
At the moment, it is a tri-boot system with Windows XP, 7, and Linux. Working on adding OS X to it as well.
I have a Dell Latitude D620 that does the job, but the screen washes out in bright sunlight. This particular model was one of the last Dell laptops available with an RS-232 serial port so I don't have to mess with USB-to-serial converters in order to connect the computer to whatever APRS device I'm using.
In have a 'Gateway' in the car and a 'Dell' in the house. Both work satisfactorily. I've found that it isn't the computer so much as the peripherals (and programming) hooked to it that make a big difference. Of course, computer 'speed' and memory play a big part in that too.