Ok, figured out the problem I was having: Darkstone (not exactly the most cpu- or graphics-intense game) locks up my c450a and I was convinced my shiny new g400 had an AGP issue (since it didn't lock it at 300). Maybe not. Turn DH off, no problem. So, I got a question (and the info SHOULD be in a FAQ somewhere!) - HOW MANY games have a problem when DH is enabled? How can you tell which ones are likely to? What are the odds a game problem is DH-related? etc...