Your game engine should
always come first. Until you know what the limitations are of the control system, and design there is no way to be able to optimise the graphics.
Quote: "Graphics are the most important to me then gameplay in second. Thats why I bought halo2. Halo2=best graphics in the freakin world!!!!!!!!! "
Halo 2 looks crap, and plays even worse. Sorry but quite frankly it is a typical example of what NOT to do developing a game. If it hadn't been the sequal to Halo, it would've had aweful reviews and very few people would've actually bought it.
Also considering the genres are completely different you can get away with different things. You screw an RPG Gamers over on gameplay you will promptly be linched at ANX or such

Seriously have you ever played a REAL FPS like Perfect Dark after playing Halo 2, Half-Life 2, or Doom 3?
Wow they all have amazing graphics, and you know what... a 6year old game
still is easily much better than they hope to be.
If it hadn't been an N64 exclusive, it would've become a far bigger game that it is now.
You know why it was so good?
Because it had the most advanced graphics the N64 had ever seen?
No, because it had inherited and extended the Gameplay that had been established in Goldeneye. There are so many nuanices that just make the game feel so much more alive than even these top titles with Rag-Doll Physics.
How many games do you know where you shoot someone in the leg and not only will they limp from then on, but it will affect how quickly they will be able to attack you, even fall over and have to get back up?
How many games do you know where the enemy will go to shoot you, only for thier weapon to jam.. swear at it like a salior while you can then blow them away?
Or how about the fact that you have realistic objectives on your missions, such'as having to reprogram a taxi cab in order to make it crash in to a wall so you can distracts, to sneak past a bunch of guards who lock your escape route if you come close.
Things like that have NOTHING to do with the graphics, what-so-ever. Sure the game did push the graphical bounderies that were not capable of being toppled until the next generation consoles 2years later (first by the same team), but compared to the PC at the time; the graphics weren't exactly pushing what could be done in engines like Half-Life or Quake3, simply because the model polycounts were so much lower and the UV Mapping on the N64 really tied hands quite alot.
But honestly speaking, if you just add something as simple as Field of Depth to a relatively low polygon game, it adds to the depth of the game. Can change it's look completely.
Provided ther graphics fit it doesn't matter how good they are to begin with, hell most of the time you start with a basic test graphics set so you can get in place the engine, the features you want. Then you know you can just push polycounts until you reach the wall. Allows you to optimise if you choose or have a lower systems specification then you thought.