If you experience trouble, upgrading to the next patch is extremely easy (Alec usually posts a link to a .rar download that you drag to the Aquaria folder). If the game works perfectly, I wouldn't bother (unless there's new content in the patch - e.g. artwork, features, etc.) As for the DRM - there is no DRM in the latest version. To move earlier versions from one computer to another, you need to use the message you received when you bought the game to refresh the download link. Good luck, and welcome to the forums!
BTW, admirable policy for buying games - though the Steam version is marginally more convenient, you're right about Valve's cut (though the sheer exposure Aquaria receives alone is worth it).