If your current machine is having problems with file sizes then you may not have enough ram. We run AutoCAD and Revit in our office. Most of the guys running Revit have moved to 8GB of ram which means that they have to use the 64bit version of Windows 7. I still run 4GB of ram using Windows XP and it runs fine as long as I don't try to load the real big files.
If your current laptop has less than 4GB of ram then you might just need to add some ram. If you already have 4GB then you would have to move up to more ram and a 64bit OS.
The brand of computer should not really matter so much as what hardware it contains. The exception to this is when you start running a 64bit OS. You would need good enough hardware to get good drivers for the 64bit OS. You can actually go to the autocad website and they have a database of graphics hardware that has been tested with autocad and revit.
As for brands, Dell, Lenovo and HP all make professional class machines that should run cad software, in addition to their cheaper machines that they sell retail. I have a Dell E6500 that I run Autocad and Revit on, but since I'm using the 32bit version of Windows I could probably get by with any laptop that has 4GB of ram and a reasonably fast processor.