The popular Shooter Call of Duty: Modern Warfare is an established part of our benchmark procedure. At last year's E3 it won the "Best Graphics” award and thus could beat Crysis. CoD 4 sold more copies, too, although the graphics are only DX9 based.
We benched the initial sequence of the mission "The Bog” in the first act of the game. With several comrades we run down a street while being shot at by the enemy. The visual effects mainly stress the graphics card.
I'm not sure about CoD an Battlefield (never interested in the series anyway, now totally lost in dozens of sequels), but that Starcraft2 requires a bit more graphics power, then years old AoE3 or even WoW&W3 (which are as old and had outdated graphics even when brand new) shouldn't be a suprise.
I can't see any reason of getting 9400 GT (unless where you buy has weird pricing). You can get 9500 GT for the same price (even with 20$ rebate). And even better just invest another 15$ and get yourself 4670 or 9600GT.
But even with 9500GT you can run COD5 fine on Max up to 1280 resolution.