Just about any new laptop will have the ability to run DirectRT without a problem. There are however a few upgrades that can definitely help out the performance of your computer and application.
When running DirectRT, it is recommended that you have a dedicated graphics card. This should be the most important factor in a laptop that you purchase. Although smaller laptops such as the ASUS Eee net-book PC have the power to run DirectRT, they can occasionally slow down.
With that in mind, just about any laptop that -isn't- a net-book should blaze through the software. The following specs would be more than enough for DirectRT:
The above setup will have no problems running even the 2010 release of DirectRT.Display - 800 x 600, 256 colors (Minimum); 1024 x 768 high color, 32-bit (Recommended)
Memory - 1 GB (Minimum); 2 GB (Recommended) -- Memory is cheap these days, so I recommend getting as much as you can. 32 Bit OS's can't take more than 3 gigs of RAM.
Processor - 1 GHz Pentium processor or equivalent (Minimum); 2 GHz Pentium processor or equivalent (Recommended) You shouldn't need a multi-core processor. Just make sure you aren't running too many programs at once during testing.
Graphics Card - 32 MB's (Minimum); 256 MB's (Recommended).