If you buy a Download version, you are allowed to burn the downloaded file on a blank DVD-rom (for personal use, of course), so you can install the game later on another computer - I add my personnal advice, that laptops are not the best choice to run 3D games, it will be either very expensive, or not enough powerful to run games with a high quality/performance ratio.
You still can keep the download installation files on another external storage - USB key, external hard disk, or whatever. And, as Tomaten said, If you buy the download version, you'll be able to redownload the game as many times as you want.