Stress is one thing, but if you want to check your system out for EYE CANDY, then here is my slate:
Half Life 2 - This game was the original reason I bought a new system. Crank it up, see what your system will do with it. Plus, who can resist fragging someone online with the gravity gun?
Far Cry - Not the newest game under the sun, but man oh man what a nice one. Lighting and distance are the major cool factors here. Get up to a good vantage point and look at the world around you: if your video card had a proverbial jaw, it would hit the proverbial floor.
FEAR - Creepy, very friggin' creepy. Get a high end system, and you'll crap your pants when those lights start to flicker in the game... well, at least I did.
The ambiant visual effects should be some nice eye candy to slice thru.
Serious Sam 2 - Fun factor is high, and if your screen res is too, then this would make for some serious eye candy.
Elder Scrolls IV - This is the reason I have an ATI X1900XTX showing up at my doorstep tomorrow. My GOD this game will push a system. Good for both eye candy AND stress test.