Any video card's level of detail capabilities will depend upon each individual game/title and the resolution (which you did not provide) being used to play it. An i5-2500K + 5670 combo may actually struggle to max out GTAIV at resolutions above 1366x768, but it could probably max out CoD:MW at 1680x1050.
If you truly want to play games using high detail settings, then you should be looking at more powerful cards than either of these. I suggest the HD6850 / GTX 460 range.
Your 2500k will be not even breaking a sweat with the video cards you listed. Either video card will be the limiting factor in video games. However, with the age and technology in the games you listed, I would think a 5670 would be perfect to run those games on high settings at 1680x1050 and will probably hold you by until the next gen GPUs come out.