Yep, you can redownload it any time, and even have it installed on two computers at once if you own them both. If you order a CD by mail you should receive your serial key immediately so you can download it while you wait. Note though that the DA CD will just be a disc with the pdf manual on it, not a full retail box, so if you can download the game fine you're just as well off making your own disc via SDC's archive option. If you'd like a paper manual and all that goodness, you'll need to get GC2:Gold.

They will be able to step you through everything you need to do, then. Make sure you received an automated response to your email with information for checking on your support ticket, if you did not then your email may not have been received.