Hard OCP Enthusiast has made some quite extensive testing on Hellgate: London on different graphics cards and looked at the difference between DX9 and DX10 in terms of performance. They seem over all positive on the performance of the game, and goes into detail on how different video cards hold up.
"Hellgate: London is just fun. It is what computer games are supposed to be. It looks fantastic, performs beautifully, and it offers something for almost anyone. It is an expertly crafted hybrid of First-Person Shooter, of Role-Playing Game, and of MMO. Hellgate kept us on our toes throughout the gameplay experience by being different literally every time we played it. It probably isn't the game that will make anyone want to upgrade from XP to Vista, but it may just make you glad if you already have Windows Vista and a DirectX 10 capable video card."