3G downloading a retina-only game? If I try to download a game from the AppStore from an iPhone 3G, and the game says in the requirements that it only supports 3GS+, will the AppStore immediately disallow me the download or will it let me download nonetheless?
No, it will complete the purchase request regardless. It just won't run on your device.

If you did make a purchase that doesn't run on your current device, you will have to contact the iTunes team and request a refund.