Judging by GTA4, graphics is a very important aspect for Rockstar. Maybe they will go with top-down view and make graphics look incredibly detailed and crisp instead of standard third person view with blocky and choppy 3D models.. Also, graphics in games is never a deciding factor for success... --- "Unless its a farm!!"
I think it will be well planned and turn out to be a success, the DS is such a popular system, I can't see them putting a game that could potentially sell millions on the system if it isn't any good. We'll see in a few months though.
As for graphics, I have never played a good game that has bad graphics, could you name a few?